Crash report dialog for non-opted-in users

Follow-up to #364 / PR #372.

### Problem

Legacy Desktop shows a "Send crash report?" dialog via Sentry when the app crashes, even for users who have not opted into telemetry. Desktop 2.0 silently drops everything when telemetry is disabled. This means crashes from non-opted-in users are completely invisible.

### Proposal

When the app encounters a fatal error and telemetry is disabled, show a dialog:

**Title:** "ComfyUI Desktop encountered an error"
**Body:** "Would you like to send an anonymous crash report to help us fix this issue? No personal information will be included."
**Buttons:** "Send Report" / "Don't Send"

If the user clicks "Send Report":
- Temporarily grant Datadog tracking consent for this single report
- Send the error event with system info context
- Revoke consent immediately after

Alternatively, the crash report could be written to a local file that the user can manually share (ties into #376 diagnostic report).

### PII considerations

- Apply all existing `scrubPII()` rules
- The dialog should clearly state what data will be sent
- This is a per-incident opt-in, not a blanket consent change