Add an About section to the account menu (#51)

Shows the app version, links the AGPL-3.0-or-later license text, and
links the source repo -- AGPL's own suggested-usage text recommends
exactly this ("if your software can interact with users remotely...
its interface could display a 'Source' link"), not just a courtesy
credits screen.

Version comes from package.json at build time via a Vite `define`
(__APP_VERSION__), so it can't drift from what's actually released.
License text is served at /LICENSE via a frontend/public/ symlink to
the repo-root LICENSE, the same pattern already used for the user
guide. Also bumps both package manifests to 1.0.0 ahead of tagging
the first release.

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

backend/pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "ds-chat"
-version = "0.1.0"
+version = "1.0.0"
 description = "DS Chat backend service"
 license = { text = "AGPL-3.0-or-later" }
 requires-python = ">=3.11"