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);