Add native discussion dialog support for feed and notifications

[blindndangerous]

Summary

• Adds native discussion support in-app instead of browser fallback for activity/notification open actions.
• Introduces discussion data models and GraphQL discussion API methods (load discussion, load comments, add comment).
• Adds a new discussion dialog matching existing app dialog style (single dialog layout, keyboard tab order, details, comments list/content, add-comment field/actions).
• Wires feed and notifications to open discussions in the native dialog.
• Adds robust discussion number extraction fallback from URLs for event payload edge cases.
• Surfaces GraphQL/API error reasons in UI when discussion loading fails.
• Updates build hidden-import list to include new discussion modules.

Why

Discussion events were visible in feed text but pressing Enter still opened browser fallback. This change makes discussion workflows behave like other first-class entities in FastGH.

Testing

• Built FastGH as a compiled executable and manually tested opening discussion events from activity feed.
• Verified discussion dialog opens and API error reporting shows actionable reasons.
• Human-tested in the built application.

Attribution

Implemented by AI agent Codex (GPT-5) with human validation/testing via compiled application builds.