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/Modal.css

@@ -429,6 +429,17 @@
   color: var(--ds-muted);
 }
 
+.custom-emoji-upload-preview {
+  display: block;
+  width: 64px;
+  height: 64px;
+  object-fit: contain;
+  margin-top: var(--sp-2);
+  background: var(--ds-surface-2);
+  border-radius: var(--radius);
+  border: 1px solid var(--ds-border);
+}
+
 .modal-list-row-action {
   flex: none;
   background: transparent;