feat(viewer): render github title source links

[dutifulbob]

Summary

• add a source URL classifier for title source links
• render GitHub source links with the checked-in GitHub logo
• show #number for GitHub issue and PR URLs
• keep Discord and generic external link behavior intact

Validation

• pnpm test -- StoryDetailPanel.test.tsx sourceLinks.test.ts
• pnpm typecheck
• pnpm build

[dutifulbob]

Final report:

Implemented GitHub title source link rendering through the shared title source-link boundary.

Validation passed:

• pnpm test -- StoryDetailPanel.test.tsx sourceLinks.test.ts
• pnpm typecheck
• pnpm build
• codex review --base main: no actionable issues
• CI: Backend (Go) passed, Frontend (TypeScript) passed

Behavior:

• Discord title links keep the existing Discord message treatment.
• GitHub issue and PR URLs render with the GitHub logo plus #number.
• Generic GitHub URLs render as logo-only title actions.
• Generic external URLs keep the external-link icon.