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/NewSessionModal.tsx

@@ -52,7 +52,7 @@ export function NewSessionModal({
       <div className="w-full max-w-md rounded-xl border border-zinc-800 bg-zinc-900 shadow-2xl">
         {/* Header */}
         <div className="flex items-center justify-between border-b border-zinc-800 px-5 py-4">
-          <h2 className="text-sm font-semibold text-zinc-100">New Session</h2>
+          <h2 className="text-[13px] font-semibold text-zinc-100">New Session</h2>
           <button
             className="flex size-7 items-center justify-center rounded-lg text-zinc-500 transition hover:bg-zinc-800 hover:text-zinc-300"
             onClick={onClose}