chore: upgrade to Next 16 / React 19.2.3 + repo consistency parity

[RayderC]

Part of a cross-repo consistency pass across AstroFit / Amethyst / ComicOrbit / SkyBit. SkyBit was the version outlier, so this PR also brings it onto the siblings' framework versions. No breaking changes to deployments — the session cookie name, port (7070), and volume paths are untouched, and the session-secret change is backward-compatible (see below).

Changes in this repo

• Framework upgrade — Next 15.3.3 → 16.1.1, React/React-DOM 19.1.0 → 19.2.3, plus eslint-config-next 16.1.1. Migration fixes: replaced 5 <a href> page links with <Link>, fixed 2 JSX comment text-nodes, and added a flat eslint.config.mjs (since next lint is removed in Next 16). Async params/cookies() usage was already correct.
• SESSION_SECRET alias (backward-compatible) — the app and docker-entrypoint.sh now accept SESSION_SECRET (the standard name across the other three apps) and still honor the existing SECRET_KEY. Existing deployments using SECRET_KEY keep working unchanged.
• Parity additions — CLAUDE.md, typecheck script, .env.example, docker-compose.yml, .github/workflows/ci.yml, and a Docker HEALTHCHECK.

Verification

npm run lint (0 errors), npm run typecheck (clean), and npm run build (Next 16 Turbopack, 10 app routes + 34 API routes) all pass.

⚠️ Recommend a quick manual smoke test of the file browser / upload / share flows before merging, since this includes a framework bump.

https://claude.ai/code/session_013yX3FPohZkQNiUGSNwwjbM

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