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/EmojiPicker.css

@@ -69,6 +69,26 @@
   padding: 4px 4px 2px;
 }
 
+.emoji-picker-category-label-row {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+}
+
+.emoji-picker-add-custom {
+  background: transparent;
+  border: none;
+  color: var(--ds-accent);
+  font-size: 0.7rem;
+  font-weight: 600;
+  cursor: pointer;
+  padding: 2px 4px;
+}
+
+.emoji-picker-add-custom:hover {
+  text-decoration: underline;
+}
+
 .emoji-picker-grid {
   display: grid;
   grid-template-columns: repeat(9, 1fr);
@@ -89,6 +109,32 @@
   background: var(--ds-surface-2);
 }
 
+.emoji-picker-item-custom {
+  position: relative;
+}
+
+.emoji-picker-item-remove {
+  position: absolute;
+  top: -2px;
+  right: -2px;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  width: 14px;
+  height: 14px;
+  border-radius: 50%;
+  background: var(--ds-danger);
+  color: white;
+  font-size: 0.65rem;
+  line-height: 1;
+  opacity: 0;
+  cursor: pointer;
+}
+
+.emoji-picker-item-custom:hover .emoji-picker-item-remove {
+  opacity: 1;
+}
+
 /* 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