feat: /profile dashboard with tabbed preferences, account, bookmarks, collections (0.18.0)

Tabbed user dashboard at `/profile` that absorbs `/settings` and
surfaces bookmarks + collections in one place.

- New `/profile` shell with tabs: Overview (counts), Preferences
  (theme + reader prefs, ported from /settings; works for guests
  via localStorage), Account (password change; auth-gated),
  Bookmarks, Collections. Guest tab list is filtered to what they
  can actually use.
- `/settings` is a 308 redirect to `/profile/preferences` so old
  bookmarks land cleanly. The "Settings" link in the top nav is
  replaced by a Profile link between Upload and Bookmarks; Bookmarks
  + Collections stay as shortcuts per the user spec.
- Extracts `lib/components/BookmarkList.svelte` and
  `lib/components/CollectionsGrid.svelte` so the top-level
  /bookmarks + /collections routes and the new profile tabs render
  the same UI without duplication. Both layers use a three-state
  load (authenticated / guest / error) to handle network hiccups
  inline.
- Deep links preserved via `?next=` on every sign-in CTA.

88 frontend unit tests + svelte-check clean; 12 of 12 e2e tests in
profile.spec.ts and reader-mode.spec.ts pass (8 other e2e failures
predate this branch and stay flagged for cleanup).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/routes/profile/collections/+page.svelte

@@ -0,0 +1,32 @@
+<script lang="ts">
+    import CollectionsGrid from '$lib/components/CollectionsGrid.svelte';
+
+    let { data } = $props();
+    const collections = $derived(data.collections);
+</script>
+
+{#if data.error}
+    <p class="error" role="alert" data-testid="profile-collections-error">
+        Couldn't load collections: {data.error}
+    </p>
+{:else if !data.authenticated}
+    <p class="hint" data-testid="profile-collections-signin">
+        <a href="/login?next=/profile/collections">Sign in</a> to see your collections.
+    </p>
+{:else if collections.length === 0}
+    <p class="hint" data-testid="profile-collections-empty">
+        You don't have any collections yet. Open any manga and use
+        <strong>Add to collection</strong> to start one.
+    </p>
+{:else}
+    <CollectionsGrid {collections} />
+{/if}
+
+<style>
+    .hint {
+        color: var(--text-muted);
+    }
+    .error {
+        color: var(--danger);
+    }
+</style>