Add resizable room panel, searchable user picker, and direct room membership

Room info panel is now user-resizable (fixing a layout clip at narrow
widths), and every user-selection spot (room membership, admin ownership
transfer) uses a new searchable UserPicker instead of raw text input or
prompt(). Member rows fold role + actions into a single inline dropdown
instead of a row of buttons, so the member list stays usable as rooms grow.

Room invites (the accept/decline flow) are replaced by adding a user to a
room directly -- an admin/owner picks someone and they're a member
immediately, with a "you've been added" notification email instead of an
invite email. Drops the now-unused room_invites table.

DEPLOYMENT.md

@@ -169,7 +169,7 @@ sudo -u chatapp /srv/chatapp/backend/.venv/bin/python -m app.cli generate-vapid-
 # paste the three printed lines into /etc/chatapp/env
 ```
 
-Optional: outgoing email (admin-invited signups, room-invite notifications).
+Optional: outgoing email (admin-invited signups, room membership notifications).
 Unlike everything else on this page, SMTP is **not** configured here —
 it's set through the Admin portal's Settings tab at runtime, no redeploy or
 env file edit needed. Skipped silently (logged, not an error) until an