fix(mobile): member list no longer opens on top of every room on phones (#175)

Room.tsx showed the MembersDrawer on phone widths whenever the persisted
desktop preference isPeopleDrawer was true — which is the default — so a
new mobile session opened every room behind a full-screen member list.
Mobile now uses a transient mobileMembersPanelAtom (header ⋮ → Members
toggles it, the drawer's X closes it, leaving the room resets it); the
desktop drawer keeps using the setting. Verified with Playwright at 400px.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/state/mobileMembersPanel.ts

@@ -0,0 +1,8 @@
+import { atom } from 'jotai';
+
+// Whether the member list is open on a phone-width layout, where it is a
+// full-screen panel rather than a side drawer. Deliberately NOT the persisted
+// `isPeopleDrawer` setting: that one remembers the desktop drawer (default
+// on), and reusing it on mobile made every room open on top of the member
+// list until the user found the X (Gitea #175).
+export const mobileMembersPanelAtom = atom<boolean>(false);