Prepare for public release: de-identify docs, add contributor terms

Removes personal infra baked into copy-paste instructions and the
app's own UI ahead of an eventual public release:

- AboutModal's "Source code" link is now a build-time env var
  (VITE_SOURCE_URL) instead of a hardcoded personal Gitea URL, and
  hides itself when unset rather than pointing somewhere wrong
- DEPLOYMENT.md's clone steps are genericized to any git host
- LICENSE gets its previously-blank copyright/description lines filled in
- CONTRIBUTING.md adds a lightweight contributor-terms note to keep a
  future dual-licensed offering possible once outside PRs start arriving

Deliberately out of scope for now: git commit history (still under the
real author identity) and the actual publish destination -- both still
undecided.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/README.md

@@ -112,7 +112,7 @@ src/
     LoginPage.tsx, SignupPage.tsx, ForgotPasswordPage.tsx, ResetPasswordPage.tsx
     ChatShellPage.tsx, AdminPage.tsx, HelpPage.tsx
 
-  styles/tokens.css          design tokens (DarkSingularity theme: colors, spacing, etc.)
+  styles/tokens.css          design tokens (default theme: colors, spacing, etc.)
   sw.ts                      custom service worker (injectManifest): app-shell
                                precache + NetworkFirst runtime caching, push/notificationclick
                                handlers, SKIP_WAITING messaging for the update-prompt flow