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/.env.example

@@ -0,0 +1,6 @@
+# Shown as the "Source code" link in the app's About screen -- required
+# for AGPL-3.0 section 13 compliance once you deploy this (a link so users
+# interacting with the app over the network can get the actual source,
+# including any modifications you've made). Point this at wherever *your*
+# copy of the repo actually lives, not the upstream project.
+VITE_SOURCE_URL=https://example.com/your-org/ds-chat