Reduce terminal and dashboard memory footprint

Frontend terminal:
- Share single TextDecoder across all instances (was one per terminal)
- Write binary WS frames as Uint8Array directly to ghostty-web,
  avoiding intermediate string allocation from TextDecoder.decode()
- Reduce default scrollback to 200 on mobile (was 1000; Safari mobile
  has ~300MB budget across all tabs)
- Pause heartbeat watchdog when tab is hidden to reduce WS traffic
  and message queue growth for background terminals

Dashboard screenshots:
- Revoke old object URL before creating new one (reduces peak memory
  by not having two decoded bitmaps alive simultaneously)
- Re-lookup card from cardsBySlug in fetch callback to avoid orphaned
  blob URLs when renderTiles() runs during in-flight requests
- Remove thumbnailCache writes (was redundant with activeObjectURLBySlug)

Server:
- Halve replay buffer from 256KB to 128KB per session

webterm/server.go

@@ -1270,7 +1270,6 @@ func (s *LocalServer) handleRoot(w http.ResponseWriter, r *http.Request) {
 				return;
 			}
 			screenshotRequestInFlight = true;
-			const img = card.img;
 			const url = '/screenshot.svg?route_key=' + encodeURIComponent(slug);
 			const controller = new AbortController();
 			const timeout = setTimeout(() => controller.abort(), 5000);
@@ -1290,14 +1289,13 @@ func (s *LocalServer) handleRoot(w http.ResponseWriter, r *http.Request) {
 				})
 				.then((blob) => {
 					if (!blob) return;
+					const currentCard = cardsBySlug[slug];
+					if (!currentCard || !currentCard.img) return;
 					const previous = activeObjectURLBySlug[slug];
+					if (previous) URL.revokeObjectURL(previous);
 					const objectURL = URL.createObjectURL(blob);
 					activeObjectURLBySlug[slug] = objectURL;
-					img.src = objectURL;
-					thumbnailCache[slug] = { src: objectURL, updatedAt: Date.now() };
-					if (previous) {
-						URL.revokeObjectURL(previous);
-					}
+					currentCard.img.src = objectURL;
 				})
 				.catch(() => {})
 				.finally(() => {