refactor(i18n): route git + diff panel labels through the catalog (T-466)

Migrate the user-facing strings in builtin.git (panel, status groups, commit
bar, branch picker, discard dialog, row a11y) and builtin.diff (toolbar,
empty states, file meta) to ClideSettings.i18n.string/.interpolated; extend
both catalogs. _stateLabel threaded a BuildContext from its caller. No en_US
behaviour change (D-21).

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

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

@@ -1,3 +1,15 @@
 {
-  "tab.title": { "translation": "Diff" }
+  "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" }
 }

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

@@ -1,3 +1,38 @@
 {
-  "tab.title": { "translation": "Git" }
+  "tab.title": { "translation": "Git" },
+  "panel.semantics": { "translation": "git panel" },
+  "filter.hint": { "translation": "Filter changes…" },
+  "status.loading": { "translation": "Loading…" },
+  "status.clean": { "translation": "Nothing to commit, working tree clean." },
+  "group.conflicts": { "translation": "Merge conflicts" },
+  "group.staged": { "translation": "Staged" },
+  "group.changes": { "translation": "Changes" },
+  "group.untracked": { "translation": "Untracked" },
+  "action.unstageAll": { "translation": "Unstage all" },
+  "action.stageAll": { "translation": "Stage all" },
+  "commit.message.semantics": { "translation": "commit message" },
+  "commit.button": { "translation": "Commit" },
+  "commit.button.semantics": { "translation": "commit staged changes" },
+  "branch.detached": { "translation": "(detached)" },
+  "action.pull": { "translation": "Pull" },
+  "action.pull.semantics": { "translation": "git pull" },
+  "action.push": { "translation": "Push" },
+  "action.push.semantics": { "translation": "git push" },
+  "state.added": { "translation": "added" },
+  "state.modified": { "translation": "modified" },
+  "state.deleted": { "translation": "deleted" },
+  "state.renamed": { "translation": "renamed" },
+  "state.copied": { "translation": "copied" },
+  "state.untracked": { "translation": "untracked" },
+  "row.stage.semantics": { "translation": "stage {name}" },
+  "row.unstage.semantics": { "translation": "unstage {name}" },
+  "row.discard.semantics": { "translation": "discard changes to {name}" },
+  "discard.title": { "translation": "Discard changes?" },
+  "discard.body": { "translation": "Unstaged changes to {name} will be permanently lost." },
+  "button.cancel": { "translation": "Cancel" },
+  "button.discard": { "translation": "Discard" },
+  "branch.switch.semantics": { "translation": "switch branch — {branch}" },
+  "branchPicker.title": { "translation": "Switch branch" },
+  "branchPicker.empty": { "translation": "No branches found." },
+  "branchPicker.loadFailed": { "translation": "failed to load branches" }
 }