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/ChatPane.tsx

@@ -356,7 +356,7 @@ export function ChatPane({
         {session.messages.length === 0 ? (
           <div className="flex h-full flex-col items-center justify-center gap-2 px-6 text-center">
             <Bot className="size-10 text-zinc-800" />
-            <p className="text-sm text-zinc-500">Send a message to start the conversation</p>
+            <p className="text-[13px] text-zinc-500">Send a message to start the conversation</p>
             <p className="text-[12px] text-zinc-700">
               {isScratchpad ? (
                 <>