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/routes/collections/[id]/+page.svelte

@@ -12,6 +12,7 @@
     import type { Manga } from '$lib/api/client';
     import { fileUrl } from '$lib/api/client';
     import MangaCard from '$lib/components/MangaCard.svelte';
+    import IconButton from '$lib/components/IconButton.svelte';
     import ArrowLeft from '@lucide/svelte/icons/arrow-left';
     import Pencil from '@lucide/svelte/icons/pencil';
     import Check from '@lucide/svelte/icons/check';
@@ -148,26 +149,23 @@
     {:else}
         <div class="title-row">
             <h1 data-testid="collection-name">{collection.name}</h1>
-            <button
-                type="button"
-                class="icon-btn"
+            <IconButton
                 onclick={startEdit}
                 aria-label="Edit collection"
                 title="Edit"
                 data-testid="collection-edit-open"
             >
                 <Pencil size={16} aria-hidden="true" />
-            </button>
-            <button
-                type="button"
-                class="icon-btn danger"
+            </IconButton>
+            <IconButton
+                variant="danger"
                 onclick={onDeleteCollection}
                 aria-label="Delete collection"
                 title="Delete"
                 data-testid="collection-delete"
             >
                 <Trash2 size={16} aria-hidden="true" />
-            </button>
+            </IconButton>
         </div>
         {#if collection.description}
             <p class="description" data-testid="collection-description">
@@ -418,26 +416,4 @@
             height: 32px;
         }
     }
-
-    .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 {
-        background: var(--surface-elevated);
-        color: var(--text);
-    }
-
-    .icon-btn.danger:hover {
-        color: var(--danger);
-    }
 </style>