feat(i18n): localize settings + search screens

Extend the en/zh localization to the two screens reached from the home
app bar (the "全都用 + 多语言" ask):
- settings: title, theme-mode segments (System/Light/Dark), seed-color
  description, color-picker + clear-data dialogs.
- search: hint, error, empty/no-results states, Notes/Documents section
  headers, page label.

New ARB keys regenerated; l10n_test now asserts the new keys resolve in
both English and Chinese.

flutter analyze: 0 issues. l10n_test: 3/3.

lib/l10n/app_en.arb

@@ -15,6 +15,31 @@
   "openInSplitView": "Open in Split View",
   "splitViewSubtitle": "PDF reference + scratchpad",
   "removeDocument": "Remove document",
+  "ok": "OK",
+  "pickColor": "Pick a color",
+  "clearSettingsTitle": "Clear all local settings?",
+  "clear": "Clear",
+  "settingsReset": "Settings reset to defaults",
+  "themeSystem": "System",
+  "themeLight": "Light",
+  "themeDark": "Dark",
+  "seedColorDesc": "Seed color for Material 3 theme",
+  "searchHint": "Search notes and documents...",
+  "searchError": "Search error: {error}",
+  "@searchError": {
+    "placeholders": { "error": { "type": "String" } }
+  },
+  "noResultsFor": "No results for \"{query}\"",
+  "@noResultsFor": {
+    "placeholders": { "query": { "type": "String" } }
+  },
+  "typeToSearch": "Type to search your notes and documents",
+  "sectionNotes": "Notes",
+  "sectionDocuments": "Documents",
+  "pageLabel": "Page {page}",
+  "@pageLabel": {
+    "placeholders": { "page": { "type": "int" } }
+  },
   "processingPptx": "Processing PPTX...",
   "processingPresentation": "Processing presentation...",
   "couldNotOpenPresentation": "Could not open presentation.",