feat: let users change their auto-generated username in Settings

Usernames are auto-assigned at signup (shipped earlier this session)
but there was still no way to change one. Adds a Username field to
Settings, validated client- and server-side against the same
3-20-char lowercase/digits/underscore pattern used for generation,
with a 409 on collision (excluding the user's own current username).

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

apps/web/messages/en.json

@@ -1133,6 +1133,10 @@
     "avatarUploadFailed": "Failed to update profile photo",
     "avatarRemoved": "Profile photo removed",
     "displayName": "Display name",
+    "username": "Username",
+    "usernameDescription": "Your profile URL and how people find you when following or mentioning you.",
+    "usernameInvalid": "3-20 characters: lowercase letters, numbers, and underscores only.",
+    "usernameTaken": "That username is already taken.",
     "email": "Email",
     "emailReadOnly": "Email cannot be changed here.",
     "saving": "Saving…",