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/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>