fix(i18n): close theme-picker + diff catalog gaps (T-463)

theme-picker referenced section.appearance + toggle.highContrast with no
catalog entries (silently fell to the placeholder); add both. builtin.diff
contributes a localized tab but shipped no catalog file at all; create it with
tab.title. Both namespaces now resolve from the catalog.

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

lib/kernel/src/i18n/catalog/builtin.diff_en_us.json

@@ -0,0 +1,3 @@
+{
+  "tab.title": { "translation": "Diff" }
+}

lib/kernel/src/i18n/catalog/builtin.theme-picker_en_us.json

@@ -3,5 +3,7 @@
   "modal.title": { "translation": "Select theme" },
   "modal.cancel": { "translation": "Cancel" },
   "modal.cancel.hint": { "translation": "Close the theme picker without changing the current theme" },
-  "row.select.hint": { "translation": "Activate this theme" }
+  "row.select.hint": { "translation": "Activate this theme" },
+  "section.appearance": { "translation": "Appearance" },
+  "toggle.highContrast": { "translation": "High contrast" }
 }