chore: add robots.txt, update docs with mobile layout fix

- Add frontend/public/robots.txt: allow public pages, block auth/user-private routes and /api/
- ROADMAP.md: add mobile layout fix to UX & Polish shipped items
- TROUBLESHOOTING.md: add section on Android Chrome viewport overflow root cause and fix

Authored by: Jack Levy

frontend/public/robots.txt

@@ -0,0 +1,18 @@
+User-agent: *
+Allow: /
+Allow: /bills
+Allow: /bills/
+Allow: /members
+Allow: /members/
+Allow: /topics
+Allow: /how-it-works
+Allow: /share/
+
+Disallow: /login
+Disallow: /register
+Disallow: /settings
+Disallow: /notifications
+Disallow: /following
+Disallow: /collections
+Disallow: /alignment
+Disallow: /api/