Fix mobile display: RoomInfoPanel inaccessible, emoji picker overflow

RoomInfoPanel was hard-gated behind !isMobile in ChatShellPage.tsx, but
its trigger button ("Room details" in ChatPane.tsx) still rendered and
toggled state unconditionally -- tapping it on mobile did nothing
visible, with no way to reach room info/members/settings at all. Fixed
by removing the gate and making the panel itself responsive: it renders
as a full-screen fixed overlay below the mobile breakpoint instead of
the desktop resizable aside (which stays exactly as before -- verified
in-browser at both viewport sizes).

Also found and fixed a second real bug during the mobile audit: the
emoji picker's fixed 320px width overflows a 375px-wide viewport by 5px
depending on trigger position (e.g. the composer's emoji button, near
the left edge). Shrunk it to 280px with a proportionally reduced column
count below 480px, rather than attempting dynamic position-aware sizing
for a 5px overflow.

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

frontend/src/components/EmojiPicker.css

@@ -88,3 +88,20 @@
 .emoji-picker-item:hover {
   background: var(--ds-surface-2);
 }
+
+/* The picker is positioned absolutely relative to its trigger button, which
+   can sit close enough to a narrow viewport's edge that the full 320px
+   width runs off-screen (e.g. the composer's emoji trigger, near the left
+   edge, overflows the right edge on a 375px-wide phone). Shrinking the
+   fixed width -- rather than trying to dynamically reposition based on the
+   trigger's actual offset -- is enough margin for every real trigger
+   position in this app. */
+@media (max-width: 480px) {
+  .emoji-picker {
+    width: min(280px, calc(100vw - 32px));
+  }
+
+  .emoji-picker-grid {
+    grid-template-columns: repeat(7, 1fr);
+  }
+}

frontend/src/components/RoomInfoPanel.css

@@ -28,6 +28,23 @@
   background: color-mix(in srgb, var(--ds-accent) 40%, transparent);
 }
 
+/* Below MOBILE_BREAKPOINT (frontend/src/hooks/useWindowWidth.ts), the
+   resizable-aside layout doesn't fit -- shown as a full-viewport overlay
+   instead. RoomInfoPanel.tsx skips its inline `style={{ width }}` on
+   mobile so this width rule isn't fighting an inline style, which would
+   otherwise win regardless of this media query's specificity. */
+@media (max-width: 859px) {
+  .room-info-panel {
+    position: fixed;
+    inset: 0;
+    z-index: 50;
+    width: 100%;
+    min-width: 0;
+    max-width: none;
+    border-left: none;
+  }
+}
+
 .room-info-header {
   display: flex;
   justify-content: space-between;

frontend/src/components/RoomInfoPanel.tsx

@@ -20,6 +20,7 @@ import {
 } from '../api/webhooks'
 import { useAuth } from '../context/AuthContext'
 import { useResizableWidth } from '../hooks/useResizableWidth'
+import { MOBILE_BREAKPOINT, useWindowWidth } from '../hooks/useWindowWidth'
 import type {
   EventSubscription,
   EventType,
@@ -57,6 +58,8 @@ export function RoomInfoPanel({
 }: RoomInfoPanelProps) {
   const { user } = useAuth()
   const myRole = room.role
+  const windowWidth = useWindowWidth()
+  const isMobile = windowWidth < MOBILE_BREAKPOINT
   const { width, startResize } = useResizableWidth({
     storageKey: 'room-info-panel-width',
     defaultWidth: 260,
@@ -231,8 +234,8 @@ export function RoomInfoPanel({
   }
 
   return (
-    <aside className="room-info-panel" style={{ width }}>
-      <div className="room-info-resize-handle" onPointerDown={startResize} />
+    <aside className="room-info-panel" style={isMobile ? undefined : { width }}>
+      {!isMobile && <div className="room-info-resize-handle" onPointerDown={startResize} />}
       <div className="room-info-header">
         <span className="room-info-header-label">Details</span>
         <button type="button" className="room-info-close" onClick={onClose} aria-label="Close">

frontend/src/pages/ChatShellPage.tsx

@@ -106,7 +106,7 @@ export function ChatShellPage() {
             )
           ))}
 
-        {!isMobile && infoOpen && activeRoom && (
+        {infoOpen && activeRoom && (
           <RoomInfoPanel
             room={activeRoom}
             members={members}