feat: Gravatar opt-in (off by default), configurable in Settings

Previously every account without a custom avatar automatically got
its email MD5-hashed and sent to gravatar.com at signup, with no way
to turn it off. Adds users.useGravatar (default false): removed the
automatic signup-time lookup entirely, and "remove photo" now falls
back to the initials placeholder instead of silently re-deriving a
Gravatar URL. New toggle in Settings -> Profile, off by default,
description explains the MD5-hash-to-third-party tradeoff. Existing
accounts' current avatarUrl is left untouched either way — no
retroactive avatar changes for anyone already using one.

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

apps/web/messages/en.json

@@ -1151,6 +1151,8 @@
     },
     "profile": "Profile",
     "changePhoto": "Change photo",
+    "useGravatar": "Use Gravatar",
+    "useGravatarDescription": "Show a Gravatar photo when you haven't uploaded one — sends an MD5 hash of your email to gravatar.com. Off by default.",
     "removePhoto": "Remove photo",
     "avatarUploadSuccess": "Profile photo updated",
     "avatarUploadFailed": "Failed to update profile photo",