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.

backend/app/schemas/user.py

@@ -25,3 +25,17 @@ class UserRead(BaseModel):
 
 class ProfileUpdate(BaseModel):
     display_name: str | None = Field(default=None, max_length=50)
+
+
+class UserDirectoryRead(BaseModel):
+    """Lightweight entry for user-picker UIs (room invites, admin ownership
+    transfer) -- same visibility level as an avatar: any authenticated user
+    can see this much about anyone (excludes bots, which aren't invited
+    through these flows)."""
+
+    model_config = ConfigDict(from_attributes=True)
+
+    id: uuid.UUID
+    username: str
+    display_name: str | None
+    avatar_filename: str | None