fix(git-ui): prevent large commit diffs from freezing the history tab

Harden commit diff loading/rendering so opening a very large commit no longer hangs
the browser tab.

Problem
- commit history diff viewer rendered every diff line as a React node
- very large commits could create thousands of nodes and lock the UI thread
- backend always returned full commit patch payloads, amplifying frontend pressure

Backend safeguards
- add `COMMIT_DIFF_CHARACTER_LIMIT` (500,000 chars) in git routes
- update GET `/api/git/commit-diff` to truncate oversized diff payloads
- include `isTruncated` flag in response for observability/future UI handling
- append truncation marker text when server-side limit is applied

Frontend safeguards
- update `GitDiffViewer` to use bounded preview rendering:
  - character cap: 200,000
  - line cap: 1,500
- move diff preprocessing into `useMemo` for stable, one-pass preview computation
- show a clear "Large diff preview" notice when truncation is active

Impact
- commit diff expansion remains responsive even for high-change commits
- UI still shows useful diff content while avoiding tab lockups
- changes apply to shared diff viewer usage and improve resilience broadly

Validation
- `node --check server/routes/git.js`
- `npm run typecheck`
- `npx eslint src/components/git-panel/view/shared/GitDiffViewer.tsx`

server/routes/git.js

@@ -7,6 +7,7 @@ import { queryClaudeSDK } from '../claude-sdk.js';
 import { spawnCursor } from '../cursor-cli.js';
 
 const router = express.Router();
+const COMMIT_DIFF_CHARACTER_LIMIT = 500_000;
 
 function spawnAsync(command, args, options = {}) {
   return new Promise((resolve, reject) => {
@@ -769,8 +770,13 @@ router.get('/commit-diff', async (req, res) => {
       'git', ['show', commit],
       { cwd: projectPath }
     );
-    
-    res.json({ diff: stdout });
+
+    const isTruncated = stdout.length > COMMIT_DIFF_CHARACTER_LIMIT;
+    const diff = isTruncated
+      ? `${stdout.slice(0, COMMIT_DIFF_CHARACTER_LIMIT)}\n\n... Diff truncated to keep the UI responsive ...`
+      : stdout;
+
+    res.json({ diff, isTruncated });
   } catch (error) {
     console.error('Git commit diff error:', error);
     res.json({ error: error.message });