fix(terminal): replay snapshot history at the PTY size it was drawn for

Opening the terminal panel sometimes showed stray fragments on the prompt
row: zsh's end-of-line mark and pieces of the prompt path. The shell had
laid its output out for one PTY width, but the client replayed that
history into an emulator of another width (an early size estimate, a
remount, or a renderer rebuild after fonts loaded). ghostty-web's reflow
then left fragments the shell's SIGWINCH redraw never clears.

The server now reports the PTY cols/rows in every snapshot, the transport
carries them through projections and accepted resizes, and the viewport
replays a sized snapshot chunk at that size before returning to the
fitted size. The container-based size estimate only seeds newly spawned
shells and is no longer sent to a running PTY.

Tests cover the sized replay, the store chunk size, the transport
projection, and the server snapshot; verified in a production build by
reloading with the panel open and switching tabs at a changed width.

packages/ui/src/stores/useTerminalStore.test.ts

@@ -423,6 +423,20 @@ describe('terminal state reconciliation', () => {
     expect(buffer(tabId).chunks).toBe(previous);
   });
 
+  test('records the PTY size a snapshot was drawn for and treats a size change as a new snapshot', () => {
+    const tabId = setup();
+    useTerminalStore.getState().replaceBuffer('/repo', tabId, 'prompt', 8, { cols: 94, rows: 56 });
+    expect(buffer(tabId).chunks[0].size).toEqual({ cols: 94, rows: 56 });
+    const previous = buffer(tabId).chunks;
+    useTerminalStore.getState().replaceBuffer('/repo', tabId, 'prompt', 8, { cols: 94, rows: 56 });
+    expect(buffer(tabId).chunks).toBe(previous);
+    useTerminalStore.getState().replaceBuffer('/repo', tabId, 'prompt', 8, { cols: 80, rows: 24 });
+    expect(buffer(tabId).chunks).not.toBe(previous);
+    expect(buffer(tabId).chunks[0].size).toEqual({ cols: 80, rows: 24 });
+    useTerminalStore.getState().appendToBuffer('/repo', tabId, ' live', 9);
+    expect(buffer(tabId).chunks[1].size).toBe(undefined);
+  });
+
   test('caps multibyte scrollback by UTF-8 bytes', () => {
     const tabId = setup();
     useTerminalStore.getState().appendToBuffer('/repo', tabId, '界'.repeat(200_000), 1);