fix: streaming race conditions (#631) + blank-page workspace binding (#804)

Closes #631. Closes #804.

Bug A (thinking card below answer / double render / stuck cursor): trailing rAF after 'done'
inserted a duplicate live-turn wrapper into already-settled DOM. Fixed via _streamFinalized flag
+ cancelAnimationFrame in all terminal handlers (done/apperror/cancel/_handleStreamError) +
_scheduleRender guard. All three reported symptoms were the same root cause.

Bug B (accumulator reset): original fix reset assistantText/reasoningText inside _wireSSE on reconnect.
Reverted — server uses one-shot queue.Queue(), no replay on reconnect, reset would wipe valid
pre-drop content causing data loss. Bug A fix alone resolves all symptoms.

#804 (blank page workspace): syncWorkspaceDisplays uses S._profileDefaultWorkspace as fallback;
workspace chip enabled when hasWorkspace (not hasSession); promptNewFile/promptNewFolder/
switchToWorkspace/promptWorkspacePath auto-create session on blank page; boot.js hydrates
_profileDefaultWorkspace from /api/settings before any session exists.

Opus max-effort review + Nathan independent review + full browser QA. 1765/1765 tests.

tests/test_sprint36.py

@@ -162,7 +162,7 @@ def test_sse_cancel_handler_calls_set_busy():
     if idx == -1:
         idx = src.find('addEventListener("cancel"')
     assert idx != -1
-    block = src[idx:idx + 1000]
+    block = src[idx:idx + 1200]
     assert "setBusy(false)" in block, (
         "SSE cancel handler no longer calls setBusy(false)"
     )