Nanda Town UI: audience-choice voting, live PR gallery, and site cleanup

Description

The pitch.

Brings the live nandatown.projectnanda.org UI onto the repo. All commits authored by dhve. Changes are confined to apps/nest-dashboard/ (no packages/ or scenarios/ touched), so this composes with the merged hackathon PRs already on main.

## What's in here
- **Audience-choice voting on /skills** — heart any SkillMD; counts and liker names are public, but liking needs a Google sign-in (hand-rolled OAuth, HMAC-signed session cookies, no new deps) so the $1,000 Audience Choice Award vote stays bot-free. Stored in the skill_likes table.
- **Live PR gallery at /prgallery** (renamed from /hackathon, with redirects) — every hackathon PR synced straight from GitHub: the twelve protocol layers plus an Other bucket, merged PRs pinned first. A signed GitHub webhook busts the cache the instant a PR merges; reads are cached to stay inside GitHub + DB limits.
- **Site content cleanup** — announcement banner now drives voting; homepage NandaHack callout and the "design note" pull quotes replaced with substantive Nanda Town info; Google-only sign-in; single Saturday 2 PM ET final-deadline messaging; judge-score UI removed now that judging is done.

## Note on conflicts
Four UI files (hackathon/page.tsx, page.tsx, footer.tsx, hackathon-event.ts) conflict because main edited them too. Resolve in favor of this branch's versions — they are the current live site. Everything else merges clean.

## Heads-up for maintainers
This restores the collapsible sidebar and the migrated pages that were previously reverted on main. Worth a look from @mariagorskikh before merging so we're aligned on the nav.