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.module.css

@@ -10,7 +10,7 @@
     min-height: 0;
     /* Smaller, denser chat text (cascades into user bubble + assistant markdown
        + tool cards; the explicit-size Mantine labels keep their own size). */
-    font-size: var(--mantine-font-size-sm);
+    font-size: var(--mantine-font-size-xs);
 }
 
 .messageRow {