feat(ai-chat): wider default window, larger max size, smaller font

Make the floating AI chat window open at a larger default size and
allow stretching it further, plus shrink the fonts.

- ai-chat-window.tsx: DEFAULT_WIDTH 362->540, DEFAULT_HEIGHT 602->680;
  clamp the default width to the viewport in computeInitialGeom()
  (symmetric with the existing height clamp) to avoid overflow on
  narrow screens.
- ai-chat-window.module.css: raise resize caps (max-width 560->900px,
  max-height 880->1100px); base font-size 12->11px.
- ai-chat.module.css: chat content font .messages sm->xs.

apps/client/src/features/ai-chat/components/ai-chat-window.module.css

@@ -24,9 +24,9 @@
     flex-direction: column;
     min-width: 300px;
     min-height: 400px;
-    max-width: 560px;
-    max-height: 880px;
-    font-size: 12px;
+    max-width: 900px;
+    max-height: 1100px;
+    font-size: 11px;
     color: light-dark(var(--mantine-color-black), var(--mantine-color-dark-0));
 }