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/components/MessageList.tsx

@@ -8,7 +8,7 @@ import { EMOJI_PICKER_MAX_HEIGHT, EmojiPicker } from './EmojiPicker'
 import { FilePreviewModal, getPreviewKind } from './FilePreviewModal'
 import { ImageLightbox } from './ImageLightbox'
 import { LinkPreviewCard } from './LinkPreviewCard'
-import { MessageContent } from './MessageContent'
+import { EmojiGlyph, MessageContent } from './MessageContent'
 import { UserAvatar } from './UserAvatar'
 import { VideoLightbox } from './VideoLightbox'
 import './MessageList.css'
@@ -302,7 +302,9 @@ export function MessageList({
                             title={r.user_ids.map(displayNameForUserId).join(', ')}
                             onClick={() => onReact(msg.id, r.emoji)}
                           >
-                            <span>{r.emoji}</span>
+                            <span>
+                              <EmojiGlyph value={r.emoji} />
+                            </span>
                             <span>{r.count}</span>
                           </button>
                         )