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>

LICENSE

@@ -629,8 +629,8 @@ to attach them to the start of each source file to most effectively
 state the exclusion of warranty; and each file should have at least
 the "copyright" line and a pointer to where the full notice is found.
 
-    <one line to give the program's name and a brief idea of what it does.>
-    Copyright (C) <year>  <name of author>
+    DS Chat, a self-hosted, real-time team chat service.
+    Copyright (C) 2026  Keith Smith
 
     This program is free software: you can redistribute it and/or modify
     it under the terms of the GNU Affero General Public License as published by