Move custom emoji management out of the emoji picker

The picker's delete "x" overlaps the glyph in a tightly packed grid,
which is too easy to hit by accident on a touch screen. Adding and
deleting custom emoji now live in their own modal under the account
menu, with delete gated behind the same confirm() every other
destructive action in the app uses.

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

frontend/src/components/EmojiPicker.css

@@ -69,26 +69,6 @@
   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);
@@ -109,32 +89,6 @@
   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