feat(file-history): mark latest history entry identical to working tree

Annotate file history entries with matches_working_tree and surface that
information in the UI so the most recent commit can be identified as
"current version" when its content equals the working tree.

- Backend: add FileHistoryCommit and annotate_file_history(...) which checks
  (only for regular files) whether the newest commit's blob matches the
  working tree via `git diff --quiet`. list_file_history now returns the
  annotated commits.
- Types: add optional matches_working_tree to GitCommit shape used by the UI.
- UI: show a "Current version"/"Aktueller Stand" badge and disable Diff/Restore
  actions for entries that match the working tree (text localized for de/en).

Also add a test that verifies the matching behavior across file edits,
staging, committing and deletion. No external API breaking changes.

src/lib/types.ts

@@ -260,6 +260,8 @@ export interface GitStash {
 }
 
 export interface GitCommit {
+  /** Set for the latest file-history entry when its diff against the working tree is empty. */
+  matches_working_tree?: boolean;
   hash: string;
   short_hash: string;
   summary: string;