fix: use system Chromium on unsupported OS (Ubuntu 26.04+)

Playwright's bundled browser install fails on Ubuntu 26.04 with
"Playwright does not support chromium on ubuntu26.04-x64".

Fall back to system Chromium (/usr/bin/chromium-browser etc.) via
executablePath when launching headless, bypassing the platform check.

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

src/gateway/browser-tools.ts

@@ -220,8 +220,23 @@ async function getOrCreateSession(sessionId: string): Promise<BrowserSession> {
       if (!connected) throw new Error(`Chrome launched but did not respond on port ${debugPort} after 15s. Close any existing Chrome windows and try again.`);
       console.log(`[Browser] Launched and connected to Chrome on port ${debugPort}`);
     } else {
-      console.log('[Browser] No system Chrome found; launching Playwright Chromium directly.');
-      browser = await pw.chromium.launch({ headless: false });
+      // Fall back to Playwright's bundled Chromium, but prefer system Chromium
+      // when the OS isn't officially supported by Playwright (e.g. Ubuntu 26.04+).
+      const sysCandidates = [
+        '/usr/bin/chromium-browser',
+        '/usr/bin/chromium',
+        '/snap/bin/chromium',
+      ];
+      const sysExec = sysCandidates.find(p => fs.existsSync(p));
+      if (sysExec) {
+        console.log(`[Browser] Using system Chromium: ${sysExec}`);
+      } else {
+        console.log('[Browser] No system Chrome found; launching Playwright Chromium directly.');
+      }
+      browser = await pw.chromium.launch({
+        headless: false,
+        ...(sysExec ? { executablePath: sysExec } : {}),
+      });
     }
   }