feat(viewer): UI-02 visual foundation, top bar and album info

V01+V02 (Task 2 of the UI-02 viewer redesign plan):

- viewer-tokens.css: UI-02 §3.1 design tokens scoped to .viewer-shell
  (never :root), so embed routes consume none of them
- App.vue: normal routes get a sticky white top bar (PhotoLive brand +
  compact locale switcher + lightweight "..." menu holding the
  reduce-motion toggle); global 16px content padding removed so banner
  and photos run edge-to-edge; embed shell branch untouched
- LocaleSwitcher: compact "EN/中文 + caret" trigger with a proper menu
  (current language checked), 44px touch target, Escape/outside close
- AlbumHeader (new): banner (2.4:1 mobile, 320px desktop cap,
  object-fit contain, collapses on missing/failed), 2-line expandable
  title + share button (clipboard copy with selectable-text fallback),
  3-line expandable pre-line description, meta row (Intl-formatted
  view count with em-dash fallback for unknown, album-timezone dates
  with full range on title, venue), Latest/Popular 44px tabs and
  category pills from the API with horizontal scroll + overflow fade
- ActivityPage: delegates the info area to AlbumHeader; sequence
  reset/sort-lock/category-by-id semantics unchanged; desktop content
  capped at 1120px
- i18n: viewer.* strings (brand/share/views/readMore/latest/popular/
  allCategories/noPhotos/newPhotos/reduceMotion/settings) in zh-CN+en
- tests: viewer-shell (token source + normal/embed isolation) and
  album-header (8 cases) specs; activity/i18n specs updated for the
  menu switcher and the em-dash unknown view count; e2e selectors
  refreshed

Co-Authored-By: Claude Code <noreply@anthropic.com>

apps/viewer/src/styles/viewer-tokens.css

@@ -0,0 +1,22 @@
+/*
+ * UI-02 §3.1 统一视觉 token（V01）。
+ * 作用域红线：变量只挂在 .viewer-shell（普通观众路由外壳）上——embed 路由
+ * （App.vue 的 .embed-shell 分支）不挂此类，嵌入 iframe 布局不消费任何
+ * token，亦不受本文件影响。消费方一律 var(--viewer-*)，不再散写色值。
+ */
+.viewer-shell {
+  --viewer-primary: #1A73E8;
+  --viewer-surface: #FFFFFF;
+  --viewer-page-bg: #F3F6FA;
+  --viewer-text: #172033;
+  --viewer-text-secondary: #647084;
+  --viewer-divider: #E5EAF1;
+  --viewer-dark-bg: #0E1218;
+  --viewer-like-accent: #FF667D;
+  --viewer-font-family: system-ui, -apple-system, 'Segoe UI', 'PingFang SC', sans-serif;
+  --viewer-radius-img: 4px;
+  --viewer-radius-pill: 20px;
+  --viewer-pad-info: 16px;
+  --viewer-pad-photo: 4px;
+  --viewer-touch-target: 44px;
+}