feat: show warning toast when opencode provider fails but fallback is configured

[lindixu6-hash]

Summary

When the opencode provider throws and a manual external API fallback is configured, the error was previously only logged to console. The user had no indication that their primary provider was broken and auto-capture was silently using the fallback.

This adds a non-blocking warning toast (5s, variant warning) informing the user that the opencode provider failed and the fallback is being used. The original provider error is included (truncated to 100 chars) for debugging.

Changes

• generateSummary now accepts ctx to access client.tui.showToast.
• After the opencode provider catch block, if opencodeProviderError exists AND a manual fallback (memoryModel + memoryApiUrl) is configured AND showErrorToasts is enabled, a warning toast is shown.
• The error toast path (no fallback configured) from fix: preserve opencode auto-capture errors #258 is unchanged.
• When no provider error occurs, no toast is shown (existing behavior).

Testing

• tsc --noEmit clean
• Existing auto-capture tests pass (2/2)
• Memory scope tests pass (4/4)

Follow-up from #259 review by @NaNomicon.

[lindixu6-hash]

Reviewer quick guide: This PR adds a non-blocking warning toast when the opencode provider fails but a manual fallback is configured. Previously the error was only logged, leaving users unaware their primary provider was broken.

Source diff (src/services/auto-capture.ts, ~20 lines):

• generateSummary now accepts ctx to access showToast
• After catching an opencode provider error, if a manual fallback exists AND showErrorToasts is enabled, show a 5s warning toast with the truncated error
• The error-rethrow path from fix: preserve opencode auto-capture errors #258 (no fallback configured) is unchanged
• The success path (no error) is unchanged

This addresses @NaNomicon's second follow-up from #259 review.