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/manga/[id]/edit/+page.svelte

@@ -10,6 +10,7 @@
     import { session } from '$lib/session.svelte';
     import { formatBytes, validateImageFile } from '$lib/upload-validation';
     import Chip from '$lib/components/Chip.svelte';
+    import IconButton from '$lib/components/IconButton.svelte';
     import Plus from '@lucide/svelte/icons/plus';
     import Trash2 from '@lucide/svelte/icons/trash-2';
 
@@ -190,16 +191,15 @@
                         maxlength="200"
                         data-testid="manga-author-input"
                     />
-                    <button
-                        type="button"
-                        class="icon-btn primary"
+                    <IconButton
+                        variant="primary"
                         onclick={addAuthor}
                         disabled={!authorDraft.trim()}
                         aria-label="Add author"
                         title="Add author"
                     >
                         <Plus size={16} aria-hidden="true" />
-                    </button>
+                    </IconButton>
                 </div>
             </div>
 
@@ -240,16 +240,15 @@
                         maxlength="200"
                         data-testid="manga-alt-input"
                     />
-                    <button
-                        type="button"
-                        class="icon-btn primary"
+                    <IconButton
+                        variant="primary"
                         onclick={addAltTitle}
                         disabled={!altTitleDraft.trim()}
                         aria-label="Add alternative title"
                         title="Add alternative title"
                     >
                         <Plus size={16} aria-hidden="true" />
-                    </button>
+                    </IconButton>
                 </div>
             </div>
 
@@ -270,16 +269,15 @@
                             src={fileUrl(currentCoverPath)}
                             alt="Current cover"
                         />
-                        <button
-                            type="button"
-                            class="icon-btn danger"
+                        <IconButton
+                            variant="danger"
                             onclick={markCoverForRemoval}
                             aria-label="Remove cover"
                             title="Remove cover"
                             data-testid="cover-remove"
                         >
                             <Trash2 size={16} aria-hidden="true" />
-                        </button>
+                        </IconButton>
                     </div>
                 {:else if pendingCoverRemoval}
                     <p class="hint" data-testid="cover-pending-removal">
@@ -419,39 +417,6 @@
         cursor: pointer;
     }
 
-    .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.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);
-    }
-
     .cover-preview {
         display: flex;
         align-items: flex-start;