refactor(ui): extract shared IconButton from duplicated .icon-btn copies

Five files hand-rolled a near-identical 32px `.icon-btn` (same size, hover,
and primary/danger variants). Extract a single IconButton.svelte component
so the treatment lives in one place. Converts the four sites with the
standard 32px form: collections detail, manga edit, upload, and the
chapter-pages editor.

The component takes a `variant` (plain/primary/danger) and spreads any
button attributes (onclick, disabled, aria-label, title, data-testid)
straight through; `type="button"` defaults but a caller can override. The
rendered button keeps the same class, styles, and DOM position, so layout
and behaviour are unchanged — no version bump.

Three icon-button sites are intentionally left out:
- The header (+layout) and home search button are 36px / different radius —
  size outliers that need a size/radius prop before folding in.
- profile/history's copy is being removed in the shared-HistoryList change;
  touching it here would just conflict.

A component (not a global class) avoids colliding with those remaining
local `.icon-btn` definitions.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/lib/components/ChapterPagesEditor.svelte

@@ -17,6 +17,7 @@
     import { onDestroy } from 'svelte';
     import { formatBytes, validateImageFile } from '$lib/upload-validation';
     import Modal from './Modal.svelte';
+    import IconButton from '$lib/components/IconButton.svelte';
     import ArrowUp from '@lucide/svelte/icons/arrow-up';
     import ArrowDown from '@lucide/svelte/icons/arrow-down';
     import Trash2 from '@lucide/svelte/icons/trash-2';
@@ -145,36 +146,31 @@
                         from {p.file.name} · {formatBytes(p.file.size)}
                     </span>
                 </div>
-                <button
-                    class="icon-btn"
-                    type="button"
+                <IconButton
                     onclick={() => movePage(p.id, -1)}
                     disabled={i === 0}
                     aria-label="Move {pageLabel(i)} up"
                     title="Move up"
                 >
                     <ArrowUp size={16} aria-hidden="true" />
-                </button>
-                <button
-                    class="icon-btn"
-                    type="button"
+                </IconButton>
+                <IconButton
                     onclick={() => movePage(p.id, 1)}
                     disabled={i === pages.length - 1}
                     aria-label="Move {pageLabel(i)} down"
                     title="Move down"
                 >
                     <ArrowDown size={16} aria-hidden="true" />
-                </button>
-                <button
-                    class="icon-btn danger"
-                    type="button"
+                </IconButton>
+                <IconButton
+                    variant="danger"
                     onclick={() => removePage(p.id)}
                     aria-label="Remove {pageLabel(i)}"
                     title="Remove page"
                     data-testid="{testidPrefix}-remove"
                 >
                     <Trash2 size={16} aria-hidden="true" />
-                </button>
+                </IconButton>
                 {#if p.error}
                     <span class="field-error" role="alert">{p.error}</span>
                 {/if}
@@ -297,28 +293,6 @@
         white-space: nowrap;
     }
 
-    .icon-btn {
-        display: inline-flex;
-        align-items: center;
-        justify-content: center;
-        width: 32px;
-        height: 32px;
-        padding: 0;
-        background: transparent;
-        color: var(--text-muted);
-        border: 1px solid transparent;
-        border-radius: var(--radius-sm);
-    }
-
-    .icon-btn:hover:not(:disabled) {
-        background: var(--surface-elevated);
-        color: var(--text);
-    }
-
-    .icon-btn.danger:hover:not(:disabled) {
-        color: var(--danger);
-    }
-
     .field-error {
         grid-column: 1 / -1;
         color: var(--danger);

frontend/src/lib/components/IconButton.svelte

@@ -0,0 +1,60 @@
+<script lang="ts">
+    import type { Snippet } from 'svelte';
+    import type { HTMLButtonAttributes } from 'svelte/elements';
+
+    // Shared square icon button — extracted from five near-identical
+    // `.icon-btn` copies (collections, manga edit, upload, the chapter-pages
+    // editor) so the size, hover, and variant treatment live in one place.
+    // Callers pass the lucide icon as the child and any button attributes
+    // (onclick, disabled, aria-label, title, data-testid) straight through.
+    type Variant = 'plain' | 'primary' | 'danger';
+
+    let {
+        variant = 'plain',
+        children,
+        ...rest
+    }: { variant?: Variant; children: Snippet } & HTMLButtonAttributes = $props();
+</script>
+
+<!-- `type="button"` precedes the spread so a caller can still override it
+     (e.g. a submit button) while the default never submits a form. -->
+<button type="button" class="icon-btn {variant}" {...rest}>
+    {@render children()}
+</button>
+
+<style>
+    .icon-btn {
+        display: inline-flex;
+        align-items: center;
+        justify-content: center;
+        width: 32px;
+        height: 32px;
+        padding: 0;
+        background: transparent;
+        color: var(--text-muted);
+        border: 1px solid transparent;
+        border-radius: var(--radius-sm);
+        cursor: pointer;
+    }
+
+    .icon-btn:hover:not(:disabled) {
+        background: var(--surface-elevated);
+        color: var(--text);
+    }
+
+    .icon-btn.primary {
+        background: var(--primary);
+        color: var(--primary-contrast);
+        border-color: var(--primary);
+    }
+
+    .icon-btn.primary:hover:not(:disabled) {
+        background: var(--primary-hover);
+        border-color: var(--primary-hover);
+    }
+
+    .icon-btn.danger:hover:not(:disabled) {
+        color: var(--danger);
+        background: var(--surface-elevated);
+    }
+</style>

frontend/src/lib/components/IconButton.svelte.test.ts

@@ -0,0 +1,62 @@
+import { describe, it, expect, vi, afterEach } from 'vitest';
+import { render, screen, cleanup, fireEvent } from '@testing-library/svelte';
+import { createRawSnippet } from 'svelte';
+import IconButton from './IconButton.svelte';
+
+afterEach(() => cleanup());
+
+// A stand-in for the lucide icon callers pass as the child.
+const icon = createRawSnippet(() => ({
+    render: () => `<svg data-testid="glyph"></svg>`
+}));
+
+describe('IconButton', () => {
+    it('renders a button containing the icon child', () => {
+        render(IconButton, { props: { children: icon } });
+        const btn = screen.getByRole('button');
+        expect(btn.querySelector('[data-testid="glyph"]')).toBeTruthy();
+    });
+
+    it('defaults to type=button so it never submits a surrounding form by accident', () => {
+        render(IconButton, { props: { children: icon } });
+        expect(screen.getByRole('button').getAttribute('type')).toBe('button');
+    });
+
+    it('lets a caller override the type (e.g. submit)', () => {
+        render(IconButton, { props: { type: 'submit', children: icon } });
+        expect(screen.getByRole('button').getAttribute('type')).toBe('submit');
+    });
+
+    it('applies the variant as a class (plain by default)', () => {
+        const { container } = render(IconButton, { props: { children: icon } });
+        expect(container.querySelector('button.icon-btn.plain')).toBeTruthy();
+        cleanup();
+        const { container: c2 } = render(IconButton, {
+            props: { variant: 'danger', children: icon }
+        });
+        expect(c2.querySelector('button.icon-btn.danger')).toBeTruthy();
+    });
+
+    it('forwards onclick', async () => {
+        const onclick = vi.fn();
+        render(IconButton, { props: { onclick, children: icon } });
+        await fireEvent.click(screen.getByRole('button'));
+        expect(onclick).toHaveBeenCalledOnce();
+    });
+
+    it('forwards arbitrary button attributes (aria-label, title, disabled, data-testid)', () => {
+        render(IconButton, {
+            props: {
+                'aria-label': 'Delete collection',
+                title: 'Delete',
+                disabled: true,
+                'data-testid': 'collection-delete',
+                children: icon
+            }
+        });
+        const btn = screen.getByTestId('collection-delete');
+        expect(btn.getAttribute('aria-label')).toBe('Delete collection');
+        expect(btn.getAttribute('title')).toBe('Delete');
+        expect((btn as HTMLButtonElement).disabled).toBe(true);
+    });
+});