fix: normalize oversized font sizes across the UI

Tighten font sizes to match the app's compact design language:

- WelcomePane: heading from text-xl (20px) to text-base (16px),
  buttons and descriptions from text-sm (14px) to text-[13px]
- SettingsPanel: all section headings from text-sm to text-[13px]
- NewSessionModal: title from text-sm to text-[13px]
- PatternEditor: title from text-sm to text-[13px]
- ChatPane: empty-state prompt from text-sm to text-[13px]
- Markdown prose: h1 from 1.25rem to 1.125rem, h2 from 1.125rem
  to 1rem, h3 from 1rem to 0.9375rem

Keeps text-sm only for the app brand name, loading state, and error
headings where larger text is intentional.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/renderer/components/PatternEditor.tsx

@@ -235,7 +235,7 @@ export function PatternEditor({
             <ChevronLeft className="size-4" />
           </button>
           <div>
-            <h3 className="text-sm font-semibold text-zinc-100">
+            <h3 className="text-[13px] font-semibold text-zinc-100">
               {pattern.name || 'Untitled pattern'}
             </h3>
             <p className="text-[12px] text-zinc-500">