Add emoji search by name

A search box in EmojiPicker filters the flat emoji list by name/keyword
instead of only browsing by category, shared by both the composer's
emoji trigger and the message reaction picker.

Names/keywords didn't exist anywhere in this codebase before (emoji.ts is
just raw unicode characters). Rather than hand-typing entries for all 928
unique emoji -- a lot of manual work, and unlike the codepoints themselves
a wrong name only means a bad search result, not a broken emoji, so the
accuracy argument for hand-typing didn't apply here -- generated
emojiNames.ts once from emojibase-data (MIT licensed), matched against
emoji.ts by codepoint (normalizing variation-selector differences between
the two sources). emojibase-data itself was never added to package.json;
it was only ever a one-time generation tool, so this ships with zero new
runtime dependency.

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

frontend/src/components/EmojiPicker.css

@@ -35,6 +35,31 @@
   right: 0;
 }
 
+.emoji-picker-search {
+  position: sticky;
+  top: 0;
+  z-index: 1;
+  background: var(--ds-surface-2);
+  border: 1px solid var(--ds-border);
+  border-radius: var(--radius);
+  padding: 6px 10px;
+  color: var(--ds-text);
+  font-size: 0.82rem;
+  outline: none;
+  flex: none;
+}
+
+.emoji-picker-search:focus {
+  border-color: var(--ds-accent);
+}
+
+.emoji-picker-no-results {
+  font-size: 0.82rem;
+  color: var(--ds-muted);
+  padding: var(--sp-3) var(--sp-2);
+  text-align: center;
+}
+
 .emoji-picker-category-label {
   font-size: 0.7rem;
   font-weight: 700;