refactor(i18n): bundle catalogs under assets/i18n/<locale>/ (T-462)

Move the catalogs out of lib/kernel/src/i18n/catalog/ into assets/i18n/, and
switch the layout from a `{namespace}_{locale}.json` filename to a per-locale
DIRECTORY: assets/i18n/en_us/{namespace}.json. Adding a language is now just a
new folder (assets/i18n/nl_nl/, nl_be, en_eu, …) of the same namespace files —
no renames — which is cleaner to maintain and mirrors how an external
extension ships its own catalog. AssetCatalogLoader/FileCatalogLoader resolve
`<root>/<locale>/<namespace>.json`; pubspec bundles the locale folder.

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

assets/i18n/en_us/builtin.diff.json

@@ -0,0 +1,15 @@
+{
+  "tab.title": { "translation": "Diff" },
+  "view.semantics": { "translation": "diff view" },
+  "status.loading": { "translation": "Loading…" },
+  "empty.staged": { "translation": "No staged changes." },
+  "empty.unstaged": { "translation": "No unstaged changes." },
+  "toolbar.unstaged": { "translation": "Unstaged" },
+  "toolbar.unstaged.semantics": { "translation": "show unstaged changes" },
+  "toolbar.staged": { "translation": "Staged" },
+  "toolbar.staged.semantics": { "translation": "show staged changes" },
+  "meta.newFile": { "translation": "new file" },
+  "meta.deleted": { "translation": "deleted" },
+  "meta.renamedFrom": { "translation": "renamed from {path}" },
+  "meta.binary": { "translation": "binary" }
+}