fix: add server-side crash breadcrumbs and remove --drop:console from build

The --drop:console esbuild flag was silently stripping all diagnostic
console.* calls from production builds, making crash investigation
impossible. Remove it temporarily for debugging.

Add server-side crash breadcrumb collection:
- Frontend writes breadcrumbs to localStorage AND POSTs to /api/crash-diag
- Server stores latest breadcrumbs in memory, readable via GET /api/crash-diag
- Granular breadcrumbs in selectSession: CACHE_WRITE, FETCH_START,
  FETCH_DONE, REWRITE, FOCUS, SELECT_DONE
- 2s heartbeat beacon so breadcrumbs survive tab freezes
- text/plain content-type parser for navigator.sendBeacon compatibility

Initial findings from breadcrumbs:
- Crash happens during selectSession() for sessions with large buffers
- Pattern: cached buffer exists from prior visit + live SSE data arriving
- Backend is always stable (0 crashes); this is a pure frontend issue

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/web/server.ts

@@ -630,6 +630,30 @@ export class WebServer extends EventEmitter {
       }
     });
 
+    // Crash diagnostics beacon — frontend POSTs breadcrumbs, GET to read them
+    let _crashBreadcrumbs = '';
+    this.app.addContentTypeParser('text/plain;charset=UTF-8', { parseAs: 'string' }, (_req, body, done) => {
+      try {
+        done(null, JSON.parse(body as string));
+      } catch {
+        done(null, { data: body });
+      }
+    });
+    this.app.addContentTypeParser('text/plain', { parseAs: 'string' }, (_req, body, done) => {
+      try {
+        done(null, JSON.parse(body as string));
+      } catch {
+        done(null, { data: body });
+      }
+    });
+    this.app.post('/api/crash-diag', (req, reply) => {
+      _crashBreadcrumbs = String((req.body as { data?: string })?.data || '');
+      reply.code(204).send();
+    });
+    this.app.get('/api/crash-diag', (_req, reply) => {
+      reply.code(200).send({ breadcrumbs: _crashBreadcrumbs, timestamp: Date.now() });
+    });
+
     // Register all route modules
     const ctx = this.createRouteContext();
     registerPushRoutes(this.app, ctx);