fix: suppress cancel errors and harden abort flow

[badchars]

Problem

When user presses Esc during a running session, several error handling gaps cause:

1. Stack traces shown in UI instead of silent cancellation
2. Potential process crash from unhandled rejection
3. Session stuck state if hackbrowser cleanup throws
4. Double error messages (cancel + provider error)

Fixes

1. Guard stopHackbrowser in cancel — c1b9831

If hackbrowser worker died, handle.send() throws, rest of cancel() skipped, session permanently stuck. Wrapped in try-catch.

2. Catch prompt_async rejection — 53b6f47

Fire-and-forget prompt with no .catch() causes unhandled rejection and process crash in Bun. Added .catch(() => {}).

3. Suppress cancel on sync prompt endpoint — f112e33

Same pattern as loop() fix (8aa8db8) — cancel error caught silently.

4. Skip error publishing on abort — 8e0914c

AI SDK AbortError entered catch block and published as provider error to frontend causing double error. Now checks abort.aborted first and breaks silently.

Not Fixing (by design)

• IngestQueue zombie tasks — Esc cancels only active task, queue continues. Intentional — qpause exists for stopping the queue.
• Double cancel call — defer cancel second call is harmless no-op.
• CancelledError class — string comparison works, typed error refactor deferred.
• defer+resolve+cancel race — theoretical, practically impossible timing.