fix(web): improve context menu positioning and forward UX

- render chat/message context menus via portal to document.body

- clamp menu coordinates to viewport while keeping near-cursor placement

- remove visible chat id fallbacks from chat/discover UI

- hide 'delete for everyone' checkbox for channels; show channel-specific hint

- replace forward-by-chat-id prompt with searchable chat picker modal

web/src/components/NewChatPanel.tsx

@@ -195,7 +195,7 @@ export function NewChatPanel() {
                   {discoverResults.map((chat) => (
                     <div className="mb-1 flex items-center justify-between rounded-lg bg-slate-800 px-3 py-2" key={chat.id}>
                       <div className="min-w-0">
-                        <p className="truncate text-sm font-semibold">{chat.title || `${chat.type} #${chat.id}`}</p>
+                        <p className="truncate text-sm font-semibold">{chat.display_title || chat.title || (chat.type === "group" ? "Group" : "Channel")}</p>
                         <p className="truncate text-xs text-slate-400">{chat.handle ? `@${chat.handle}` : chat.type}</p>
                       </div>
                       {chat.is_member ? (