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/components/FileHistoryDialog.svelte

@@ -13,6 +13,7 @@
     isBusy: boolean;
     isLoading: boolean;
     error?: string;
+    language?: "de" | "en";
     onDiff: (commit: GitCommit) => void;
     onRestore: (commit: GitCommit) => void;
     onClose: () => void;
@@ -24,6 +25,7 @@
     isBusy = false,
     isLoading = false,
     error = "",
+    language = "en",
     onDiff = () => {},
     onRestore = () => {},
     onClose = () => {},
@@ -86,14 +88,15 @@
                 <div>
                   <strong title={item.summary}>{item.summary}</strong>
                   <span><code>{item.short_hash}</code> · {item.author_name}</span>
+                  {#if item.matches_working_tree}<span class="current-version">{language === "de" ? "Aktueller Stand" : "Current version"}</span>{/if}
                 </div>
               </div>
               <time datetime={item.date}>{formatCommitDate(item.date)}</time>
               <div class="file-history-dialog-actions">
-                <button class="btn-sm" type="button" onclick={() => onDiff(item)} disabled={isBusy} title="Show changes against the working tree">
+                <button class="btn-sm" type="button" onclick={() => onDiff(item)} disabled={isBusy || item.matches_working_tree} title={item.matches_working_tree ? (language === "de" ? "Identisch mit der aktuellen Datei" : "Identical to the current file") : "Show changes against the working tree"}>
                   <GitCompare size={14} aria-hidden="true" /> Diff
                 </button>
-                <button class="btn-sm" type="button" onclick={() => onRestore(item)} disabled={isBusy} title="Restore this file from the selected commit">
+                <button class="btn-sm" type="button" onclick={() => onRestore(item)} disabled={isBusy || item.matches_working_tree} title={item.matches_working_tree ? (language === "de" ? "Identisch mit der aktuellen Datei" : "Identical to the current file") : "Restore this file from the selected commit"}>
                   <RotateCcw size={14} aria-hidden="true" /> Restore
                 </button>
               </div>
@@ -104,3 +107,7 @@
     </div>
   </div>
 </div>
+
+<style>
+  .file-history-dialog-commit .current-version{display:inline-flex;width:fit-content;margin-top:4px;padding:2px 6px;border:1px solid color-mix(in srgb,var(--color-accent) 25%,transparent);border-radius:4px;background:color-mix(in srgb,var(--color-accent) 8%,transparent);color:var(--color-accent);font-size:10px;font-weight:600}
+</style>