Backfill missing PR and issue titles in activity feed

[blindndangerous]

Summary

• Backfill missing titles in the Activity Feed for PR and issue events when GitHub event payloads omit them.
• Keep API usage low by only fetching missing unique (owner, repo, number) keys, batching lookups, and caching both hits and misses.
• Avoid dangling punctuation in feed text when a title is unavailable.

Details

• Added background title backfill for PullRequestEvent, PullRequestReviewEvent, PullRequestReviewCommentEvent, IssuesEvent, and IssueCommentEvent feed rows.
• Reused cached titles on subsequent refreshes to prevent repeated API calls.

Testing

• Built and tested the Windows app (FastGH.exe) from this branch.
• Manually verified feed rows now show PR/issue titles where previously missing.
• Confirmed discussion dialogs continue to work in the same build.

Stacked PR Notes

• Depends on Add native discussion dialog support for feed and notifications #2 (which depends on Improve activity feed text for issue/PR events #1).
• After Improve activity feed text for issue/PR events #1 and Add native discussion dialog support for feed and notifications #2 merge, this PR should contain only the feed-title-backfill commits.

Attribution

• Implemented by an AI agent: OpenAI Codex (GPT-5).
• Validated by human testing via application build.