Fix: Stream in gleichem Fenster öffnen + Update-Dialog Abbrechen-Button

- Stream-Tile Klick öffnet Join-Modal statt neues Fenster
- checkForUpdates optional chaining für alte Electron App
- Abbrechen-Button im Update-Check Dialog
- 15s Timeout falls Electron nicht antwortet

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

web/src/plugins/streaming/StreamingTab.tsx

@@ -611,7 +611,7 @@ export default function StreamingTab({ data }: { data: any }) {
           {streams
             .filter(s => s.id !== myStreamId)
             .map(s => (
-              <div key={s.id} className="stream-tile" onClick={() => openInNewWindow(s.id)}>
+              <div key={s.id} className="stream-tile" onClick={() => openJoinModal(s)}>
                 <div className="stream-tile-preview">
                   <span className="stream-tile-icon">{'\u{1F5A5}\uFE0F'}</span>
                   <span className="stream-live-badge"><span className="stream-live-dot" /> LIVE</span>