docs: list every integration and channel, and correct two stale rules

- README: the integrations line now names Strava, Garmin and the Health
  Connect import, the reminder channels include email and the webhook,
  and the last two feature labels say what they cover.
- UI guidelines: the password form was said to carry a hidden username
  field it does not have; the line now states when one helps.
- CONTRIBUTING: strings go into every locale file, not only English and
  German.

docs/ui-guidelines.md

@@ -252,9 +252,10 @@ failed read never falls through to "no data yet".
   input that browsers keep guessing wrong.
 - API-token and AI-provider key inputs use `autoComplete="new-password"`
   so the browser does not fill in the account password.
-- Password-change and passkey screens carry a hidden
-  `<input type="text" name="username" autoComplete="username" value={user.email} readOnly tabIndex={-1}>`
-  so password managers attribute saved credentials correctly.
+- A password form with no visible username field can carry a hidden
+  `<input type="text" name="username" autoComplete="username" readOnly tabIndex={-1}>`
+  so a password manager files the new password under the right account.
+  The password-change form in Settings has none.
 
 ### 4.5 Test buttons (every integration)