feat(web): polish master chat overlay with model selector, params, empty state, and keyboard shortcuts

- Add model selector chip/dropdown in ChatInput with 4 models (llama3.2, claude-3.5-sonnet, gpt-4o, deepseek-r1)
- Add temperature slider (0.0–2.0) and max tokens input (100–32000) via settings popover
- Persist model, temperature, and max tokens in localStorage across sessions
- Wire model/temperature/maxTokens state up through Chat.tsx to useChat hook
- Add ChatEmptyState component with greeting and 4 clickable suggested prompts
- Clicking a suggestion pre-fills the ChatInput textarea via externalValue prop
- Add Cmd/Ctrl+N and Cmd/Ctrl+L keyboard shortcuts to start new conversation
- Add "New conversation" button in ChatOverlay header with Cmd+N tooltip
- Show ChatEmptyState when conversation has only the welcome message
- Write 63 tests covering model selector, params config, empty state, and keyboard shortcuts

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/web/src/components/chat/ChatOverlay.tsx

@@ -164,6 +164,27 @@ export function ChatOverlay(): React.JSX.Element {
 
           {/* Header Controls */}
           <div className="flex items-center gap-1">
+            {/* New Conversation Button */}
+            <button
+              onClick={() => {
+                chatRef.current?.startNewConversation(null);
+              }}
+              className="rounded p-1.5 transition-colors hover:bg-black/5 focus:outline-none focus:ring-2"
+              aria-label="New conversation"
+              title="New conversation (Cmd+N)"
+            >
+              <svg
+                className="h-4 w-4"
+                style={{ color: "rgb(var(--text-secondary))" }}
+                fill="none"
+                viewBox="0 0 24 24"
+                stroke="currentColor"
+                strokeWidth={2}
+              >
+                <path d="M12 4v16m8-8H4" />
+              </svg>
+            </button>
+
             {/* Minimize Button */}
             <button
               onClick={minimize}