feat(diff): add branch scope to context panel diff view

Show every change on the current branch relative to its base in the
Changed/Staged/Last turn dropdown. The base comes from the branch's
reflog record or an explicit per-branch user choice (persisted), never
a main/master guess; when git has no record the user picks a base once
from a searchable branch list.

- server: GET /api/git/branch-base (reflog-derived base),
  GET /api/git/range-files (name-status -z with rename/copy
  destination paths and -C copy detection)
- shared UI: optional getBranchBase/getGitRangeFiles runtime APIs
  with boundary parsing; persisted per-branch overrides keyed by
  runtime+directory+branch
- DiffView: branch scope with confirmed-unavailability coercion of
  persisted tabs (detached HEAD, default-branch checkout, metadata
  settled without a default), range-invalidated diff cache guarded
  against stale completions, bounded branch-metadata retry, read-only
  diff actions in branch scope; hidden in VS Code
- helper module branchDiffScope.ts with tests for coercion,
  availability, race conditions, and retry exhaustion

packages/ui/src/lib/i18n/messages/fr.ts

@@ -1282,6 +1282,13 @@ export const dict = {
   "diffView.scope.changed": "Modifiés",
   "diffView.scope.staged": "Staged",
   "diffView.scope.lastTurn": "Dernier tour",
+  "diffView.scope.branch": "Branche",
+  "diffView.branch.resolvingBase": "Détection de la branche de base...",
+  "diffView.branch.noBaseTitle": "Aucune branche de base",
+  "diffView.branch.noBaseDescription": "Git ne conserve aucune trace de la branche d’origine de cette branche. Choisissez une branche de base pour la comparaison.",
+  "diffView.branch.loadError": "Échec du chargement des modifications de la branche",
+  "diffView.branch.loadingFiles": "Chargement des modifications de la branche...",
+  "diffView.branch.empty": "Aucune modification sur cette branche par rapport à {base}",
   "diffView.scope.selectorAria": "Sélectionner le mode de changements",
   'diffView.actions.retry': 'Réessayer',
   'diffView.actions.renderAnyway': 'Afficher quand même',