Add custom emoji support (#18)

Site-wide, any user can upload -- usable both as reactions and inline
in message text via :shortcode:, alongside the existing built-in
Unicode picker. A :shortcode: reference is stored/sent as literal
text (same as the built-in shortcode convention) and resolved to an
image at render time, so it degrades to plain text if the emoji is
later deleted.

Backend: new custom_emoji table (shortcode unique, sized to fit
MessageReaction.emoji's existing column alongside its colons), upload/
list/delete endpoints (delete restricted to uploader or site admin).

Frontend: a CustomEmojiProvider context feeds a new "Custom" category
in the emoji picker (inline upload + hover-to-remove), extends the
composer's shortcode autocomplete, and a shared EmojiGlyph resolver
renders custom emoji wherever a value can appear -- message text,
reaction pills, and the picker itself.

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

frontend/src/App.tsx

@@ -1,6 +1,7 @@
 import { Navigate, Route, Routes } from 'react-router-dom'
 import { AuthProvider, useAuth } from './context/AuthContext'
 import { ChatSocketProvider } from './context/ChatSocketContext'
+import { CustomEmojiProvider } from './context/CustomEmojiContext'
 import { AdminRoute } from './components/AdminRoute'
 import { DesktopNotificationBridge } from './components/DesktopNotificationBridge'
 import { ProtectedRoute } from './components/ProtectedRoute'
@@ -67,8 +68,10 @@ function AppRoutes() {
   if (!user) return routes
   return (
     <ChatSocketProvider key={user.id}>
-      <DesktopNotificationBridge />
-      {routes}
+      <CustomEmojiProvider>
+        <DesktopNotificationBridge />
+        {routes}
+      </CustomEmojiProvider>
     </ChatSocketProvider>
   )
 }