feat(VOC-131): add GenerateButton component and dashboard integration

[FSS3096]

• Create components/GenerateButton.tsx with all required states:

  • Idle (no repo): disabled button with muted 'Select a repo first' label
  • Idle (repo selected): primary blue CTA — 'Generate Post from Latest Activity'
  • Loading: button replaced by LoadingView spinner (no double-submit possible)
  • Error: ErrorView with retry + button reappears for retry
  • Success: stores response in sessionStorage['voca_drafts'], push('/drafts)

• Create components/DashboardClient.tsx — Client Component boundary that owns selectedRepo state and passes it to both RepoSelector and GenerateButton. This cleanly solves the Server Component → state ownership problem without prop drilling.

• Refactor components/RepoSelector.tsx to support controlled mode:

  • New optional props: onRepoSelect (callback) + selectedRepo (controlled value)
  • Controlled mode: used by DashboardClient for VOC-131 flow
  • Uncontrolled/legacy mode: original behaviour preserved for existing callers

• Update app/dashboard/page.tsx to use DashboardClient instead of bare RepoSelector; page stays a Server Component for auth guard.

Acceptance criteria covered:
✅ Button visible after repo selection ✅ Button disabled (not hidden) when no repo selected ✅ POST /api/generate called with { repoFullName } ✅ Loading state replaces button (no double-submit) ✅ Error state with retry (coordinates with #18) ✅ sessionStorage key 'voca_drafts' on success ✅ router.push('/drafts') on success ✅ Mobile: w-full, capped at max-w-[400px] centered on desktop ✅ Accessible: aria-label, aria-disabled, role=status/alert, focus-visible ring

Placeholder stubs for LoadingView and ErrorView are intentionally co-located in GenerateButton.tsx — swap to real components (#17, #18) is a one-line import change.

Relates to: VOC-131
Blocks: #17 (loading state), #18 (error handling)
Depends on: /api/generate endpoint (#15)

Summary

Type of Change

• 🐛 Bug fix

• - [ ] ✨ New feature

• - [ ] 🔨 Refactor / tech debt

• - [ ] 📝 Documentation update

• - [ ] 🚀 Performance improvement

• - [ ] 🔧 Config / tooling change

• ## What Changed

• -

• -

• ## Testing

• - [ ] Manual testing done

• - [ ] Existing tests pass

• - [ ] New tests added (if applicable)

• ## Screenshots / Demo

• ## Checklist

• - [ ] Self-reviewed the code

• - [ ] No console errors or warnings

• - [ ] PR title follows type: short description convention

• - [ ] Linked to relevant issue (closes #)

• - [ ] Ready for review (not a draft)