feat(chat): custom right-click context menus

Override the native browser menu with resource-specific menus:
- guild (rail icon): open, copy node id/name, add guild
- channel: open, join/leave, close, copy id/name, create channel
- message: copy text/id, mention author, delete (soft)
- user (member row): mention, copy id/name, move-to-bypass
  (discuss/work in-channel only), settings (self)
- blank areas (rail/channels/messages/members/root): scoped
  create/refresh actions

Menu closes on click/Escape/scroll/resize; positioned within the
viewport; the most specific target wins via stopPropagation.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/index.css

@@ -806,3 +806,61 @@ button {
 .modal-actions .btn {
   flex: 1;
 }
+
+/* ---- right-click context menu ---- */
+.ctx-menu {
+  position: fixed;
+  z-index: 1000;
+  background: var(--elevated);
+  border: 1px solid var(--border);
+  border-radius: var(--radius-sm);
+  box-shadow: 0 8px 24px rgba(0, 0, 0, 0.45);
+  padding: 6px;
+  display: flex;
+  flex-direction: column;
+  gap: 1px;
+  user-select: none;
+}
+.ctx-menu .ctx-title {
+  font-size: 11px;
+  font-weight: 600;
+  color: var(--text-faint);
+  text-transform: uppercase;
+  letter-spacing: 0.04em;
+  padding: 4px 8px 6px;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+.ctx-menu .ctx-sep {
+  height: 1px;
+  background: var(--border);
+  margin: 4px 2px;
+}
+.ctx-menu .ctx-item {
+  display: block;
+  width: 100%;
+  text-align: left;
+  background: none;
+  border: 0;
+  color: var(--text);
+  font-size: 13px;
+  padding: 7px 8px;
+  border-radius: 5px;
+  cursor: pointer;
+}
+.ctx-menu .ctx-item:hover {
+  background: var(--accent-soft);
+  color: var(--text-h);
+}
+.ctx-menu .ctx-item.danger {
+  color: var(--danger);
+}
+.ctx-menu .ctx-item.danger:hover {
+  background: rgba(242, 63, 66, 0.14);
+}
+.ctx-menu .ctx-item:disabled {
+  color: var(--text-faint);
+  cursor: not-allowed;
+  background: none;
+}