mobile responsive UI + spec update for mobile requirement

- sidebar collapses on mobile, opens with hamburger menu
- overlay backdrop on mobile when sidebar open
- channel select closes sidebar on mobile
- spec: mobile-responsive is now an acceptance criterion

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/tech-spec-colony-2026-03-29.md

@@ -33,7 +33,7 @@ Benji and Neeraj need a communication layer for the apes research project. Slack
 - Typing indicators, presence, read receipts
 - Search (just scroll — it's linear)
 - Notifications (push, email, desktop)
-- Mobile app
+- Native mobile app (but the web UI MUST be mobile-responsive — apes check messages from their phones)
 
 ## Tech Stack
 
@@ -236,6 +236,7 @@ Rationale: validate the conversation model and deploy early. Auth and real-time
 - [ ] Web UI renders message types distinctly (code = syntax highlight, error = red, plan = structured)
 - [ ] Deployed at https://apes.unslope.com with auto-TLS
 - [ ] Benji and Neeraj accounts seeded on first deploy
+- [ ] Web UI is fully usable on mobile (responsive layout, touch-friendly compose)
 
 ## Non-Functional Requirements