fix: add missing error handlers and abort cleanup in async effects

[NubsCarson]

Summary

• ChatView addImageFiles: FileReader promises lacked onerror/onabort handlers, causing promises that never settle on read failure (e.g. permission errors, corrupt files). Added .catch() on Promise.all to prevent unhandled rejections.
• ChatView preflight fetch: Added AbortController so the coding-agents preflight request is cancelled on unmount, preventing state updates on unmounted components.
• CodingAgentSettingsSection: Added AbortController for the preflight fetch and abort guards before state updates, preventing post-unmount state writes when navigating away during the 5-fetch initialization.

Test plan

• Open chat view, attach an image — verify it still previews correctly
• Quickly navigate away from chat view while preflight fetch is in-flight — verify no console warnings about state updates on unmounted component
• Open coding agent settings then navigate away quickly — verify no console warnings
• Attempt to attach a file that fails to read (e.g. revoke permissions mid-read) — verify console warning instead of unhandled rejection

🤖 Generated with Claude Code

[github-actions]

Agent Review — Service Unavailable

Both AI reviewers failed to complete the review for this run:

• Claude: failure
• Codex (fallback): failure

This is likely a temporary API outage. The PR will need to be re-reviewed once services recover.

View workflow run

6. Decision: REQUEST CHANGES (automated review unavailable — please re-trigger or wait for service recovery)