feat: add selective line restoration from historical commits

Add a new Tauri command to produce a diff between the working file and a historical commit (get_file_restore_patch) and support a new apply action ("restore-lines") that validates and applies only text-line changes for a single regular file.

Behavior changes and constraints:
- Fetch a filtered reverse diff for a file in a commit so UI can display selectable lines from an older revision.
- Applying "restore-lines" verifies the target is a regular file, rejects binary/metadata patches, and ensures the patch only modifies the selected file.
- Restored lines are applied to the working tree without staging other changes; the index is preserved.
- The operation rejects stale patches or patches targeting the wrong file.

UI wiring:
- Compare dialog gets a "Restore lines…" action for applicable modified files and opens the line-patch dialog in restore mode.
- Line-patch dialog gains a restore mode (restoreCommit) with adjusted UI/rendering to pair removed/added lines, helper text, and dedicated "Restore selected" / "Restore hunk" actions.
- App integration handles fetching the restore patch, applying selected lines, and refreshing views.

Tests:
- Add tests covering correct behavior (preserve unstaged/staged changes and index) and guard cases (stale/wrong-file patches).

src/lib/git.ts

@@ -753,3 +753,7 @@ export function submoduleAction(path: string, modulePath: string, action: "updat
 export function checkoutSubmoduleRevision(path: string, modulePath: string, revision: string, kind: "tag" | "commit"): Promise<void> {
   return invoke("checkout_submodule_revision", { path, modulePath, revision, kind });
 }
+
+export function getFileRestorePatch(path: string, commit: string, file: string): Promise<string> {
+  return invoke("get_file_restore_patch", { path, commit, file });
+}