Add direct messages (#52)

A DM is a Room with a new is_dm flag, not a separate model -- reuses
all the membership/message/WS plumbing Room already has instead of
duplicating it. The room's `name` (still required + globally unique)
is an internal, never-displayed token derived deterministically from
the two participants' sorted user IDs (dm_room_name), which makes
find-or-create a single indexed lookup and gets free race-condition
safety from the existing unique constraint -- a concurrent double-
start from both people just hits the same IntegrityError->retry-as-
lookup path create_room already established.

Both participants get the plain 'member' role (no owner/admin
distinction makes sense for a 1:1 DM), which incidentally reuses
every existing role gate to block add-member, room-settings edits,
and join-via-browse on a DM for free. update_room also gets an
explicit is_dm guard independent of that, since renaming a DM isn't
just a privacy concern -- it would silently corrupt the find-or-create
invariant. DMs are excluded from both Browse Rooms and the admin
portal's room listing (fully private, per scope).

GET /api/rooms/mine precomputes each DM's other participant (name,
avatar, presence) as dm_partner in one batched query, so the sidebar
can render a DM row without a fetch per row. Frontend: a new "Direct
Messages" sidebar section (searchable by partner name, not the
internal room name), clicking someone in the People list starts or
resumes a DM, and the chat header/composer/RoomInfoPanel all render
the partner's identity instead of a room name where it's a DM.

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

frontend/src/components/Modal.css

@@ -389,6 +389,28 @@
   border-bottom: none;
 }
 
+.modal-list-row-button {
+  width: 100%;
+  background: transparent;
+  border-left: none;
+  border-right: none;
+  border-top: none;
+  text-align: left;
+  font: inherit;
+  color: inherit;
+  cursor: pointer;
+  border-radius: var(--radius);
+}
+
+.modal-list-row-button:hover:not(:disabled) {
+  background: var(--ds-surface-2);
+}
+
+.modal-list-row-button:disabled {
+  cursor: not-allowed;
+  opacity: 0.7;
+}
+
 .modal-list-row-body {
   flex: 1;
   min-width: 0;