Add git version control panel and LAN hosting support

- Git modal (⎇ Git button): commit all diagrams as JSON to diagrams/ folder,
  push to remote, view history, restore any diagram from any past commit
- Diagrams exported to diagrams/{id}_{name}.json on each commit so diffs are
  human-readable and individual diagrams can be restored independently
- backend/run.py: changed host to 0.0.0.0 for LAN access

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

frontend/js/api.js

@@ -60,4 +60,12 @@ const api = {
     jsonUrl: (id) => `${API_BASE}/export/${id}/json`,
     formboardUrl: (id) => `${API_BASE}/export/${id}/formboard`,
   },
+  git: {
+    status:  ()                             => apiFetch("/git/status"),
+    log:     ()                             => apiFetch("/git/log"),
+    commit:  (message)                      => apiFetch("/git/commit",  { method: "POST", headers: { "Content-Type": "application/json" }, body: JSON.stringify({ message }) }),
+    push:    ()                             => apiFetch("/git/push",    { method: "POST" }),
+    history: (hash)                         => apiFetch(`/git/history/${hash}`),
+    restore: (commit_hash, filepath, name)  => apiFetch("/git/restore", { method: "POST", headers: { "Content-Type": "application/json" }, body: JSON.stringify({ commit_hash, filepath, name }) }),
+  },
 };