Scope git history to current diagram by default

- History panel defaults to "This diagram" / "All" toggle
- "This diagram" filters git log to commits that touched only that
  diagram's JSON file (git log -- diagrams/0001_*.json)
- Automatically switches to "All" when no diagram is open
- Restore flow still works per-commit, per-file

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/js/api.js

@@ -62,7 +62,7 @@ const api = {
   },
   git: {
     status:  ()                             => apiFetch("/git/status"),
-    log:     ()                             => apiFetch("/git/log"),
+    log:     (diagram_id)                   => apiFetch(diagram_id != null ? `/git/log?diagram_id=${diagram_id}` : "/git/log"),
     commit:  (message, diagram_id)          => apiFetch("/git/commit",  { method: "POST", headers: { "Content-Type": "application/json" }, body: JSON.stringify({ message, diagram_id: diagram_id ?? null }) }),
     push:    ()                             => apiFetch("/git/push",    { method: "POST" }),
     history: (hash)                         => apiFetch(`/git/history/${hash}`),