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/replay.go

@@ -2,7 +2,7 @@ package webterm
 
 import "sync"
 
-const replayBufferSize = 256 * 1024
+const replayBufferSize = 128 * 1024
 
 type ReplayBuffer struct {
 	mu    sync.Mutex

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(() => {

webterm/static/js/terminal.ts

@@ -26,6 +26,9 @@ async function getSharedGhostty(): Promise<Ghostty> {
   return sharedGhostty;
 }
 
+/** Shared TextDecoder (stateless for UTF-8, safe to share) */
+const sharedTextDecoder = new TextDecoder();
+
 /** Default font stack - prefers system monospace, falls back through programming fonts */
 const DEFAULT_FONT_FAMILY =
   'ui-monospace, "SFMono-Regular", "FiraCode Nerd Font", "FiraMono Nerd Font", ' +
@@ -660,7 +663,6 @@ class WebTerminal {
   private terminal: Terminal;
   private fitAddon: FitAddon;
   private socket: WebSocket | null = null;
-  private readonly textDecoder = new TextDecoder();
   private element: HTMLElement;
   private wsUrl: string;
   private reconnectAttempts = 0;
@@ -741,10 +743,11 @@ class WebTerminal {
     const fontFamily = config.fontFamily?.trim() || DEFAULT_FONT_FAMILY;
     const fontSize = config.fontSize ?? 16;
 
+    const defaultScrollback = isMobileDevice() ? 200 : 1000;
     const options: ITerminalOptions = {
       fontFamily,
       fontSize,
-      scrollback: config.scrollback ?? 1000,
+      scrollback: config.scrollback ?? defaultScrollback,
       cursorBlink: true,
       cursorStyle: "block",
       theme: themeToUse,
@@ -905,7 +908,12 @@ class WebTerminal {
 
     // Focus terminal when returning to the tab
     this.addTrackedListener(document, "visibilitychange", () => {
-      if (!document.hidden) {
+      if (document.hidden) {
+        this.stopHeartbeatWatchdog();
+      } else {
+        if (this.socket?.readyState === WebSocket.OPEN) {
+          this.startHeartbeatWatchdog();
+        }
         restoreFocus();
       }
     });
@@ -1634,9 +1642,8 @@ class WebTerminal {
   private handleMessage(data: string | ArrayBuffer | Blob): void {
     this.lastMessageAt = Date.now();
     if (data instanceof ArrayBuffer) {
-      // Binary data - write directly to terminal
-      const text = this.textDecoder.decode(data);
-      this.terminal.write(text);
+      // Binary data - write directly to terminal as Uint8Array (avoids string allocation)
+      this.terminal.write(new Uint8Array(data));
       return;
     }
     if (data instanceof Blob) {