feat(analysis): admin UI — reader context-menu action + dashboard section

Surfaces the scoped re-enqueue and per-page force-analyze in the UI:

- api/admin: reenqueueAnalysis({onlyUnanalyzed, mangaId, chapterId}) and
  analyzePage(pageId).
- Reader PageContextMenu gains an admin-only "Queue for analysis" item
  (canAnalyze/onAnalyzePage/analyzeState props) wired to the force-analyze
  endpoint with inline busy/done/error feedback; reset per page.
- New /admin/analysis dashboard section + nav tab: scope selector
  (whole library / one manga / one chapter), id input, and an
  "include already-analyzed" toggle (default off), with busy/notice/error.

Tests: vitest for the two clients; Playwright for the context-menu action
(admin vs non-admin) and the admin section (scope + include toggle +
validation). svelte-check + build clean; 262 vitest, 10 new e2e green.

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

frontend/src/routes/manga/[id]/chapter/[chapter_id]/+page.svelte

@@ -7,7 +7,7 @@
     import { preferences } from '$lib/preferences.svelte';
     import { updateReadProgress } from '$lib/api/read_progress';
     import { chapterLabel } from '$lib/api/chapters';
-    import { resyncChapter } from '$lib/api/admin';
+    import { resyncChapter, analyzePage } from '$lib/api/admin';
     import { readerFullscreen } from '$lib/reader-fullscreen.svelte';
     import { session } from '$lib/session.svelte';
     import Sheet from '$lib/components/Sheet.svelte';
@@ -118,6 +118,8 @@
     let actionSheetOpen = $state(false);
     let collectionsModalOpen = $state(false);
     let tagsModalOpen = $state(false);
+    // Admin-only "Queue for analysis" action feedback, reset per page.
+    let analyzeState = $state<'idle' | 'busy' | 'done' | 'error'>('idle');
 
     // Monotonically-increasing token so a slow loadPageSummary for
     // page A can't clobber a fresh load for page B. The user right-
@@ -148,13 +150,27 @@
         activePageId = pageId;
         contextMenuAnchor = anchor;
         contextMenuOpen = true;
+        analyzeState = 'idle';
         void loadPageSummary(pageId);
     }
 
+    async function handleAnalyzePage() {
+        const pageId = activePageId;
+        if (!pageId || !session.user?.is_admin || analyzeState === 'busy') return;
+        analyzeState = 'busy';
+        try {
+            await analyzePage(pageId);
+            analyzeState = 'done';
+        } catch {
+            analyzeState = 'error';
+        }
+    }
+
     function openActionSheet(pageId: string) {
         if (!session.user) return;
         activePageId = pageId;
         actionSheetOpen = true;
+        analyzeState = 'idle';
         void loadPageSummary(pageId);
     }
 
@@ -1454,6 +1470,9 @@
         onCopyLink={handleCopyLink}
         collectionsCount={activePageCollectionCount}
         tags={activePageTags}
+        canAnalyze={!!session.user?.is_admin}
+        onAnalyzePage={handleAnalyzePage}
+        {analyzeState}
     />
 
     <!-- Mobile action sheet: same two actions, larger touch targets. -->