feat: add font size and zoom settings

- Add font_size config field (10-24px, default 14px)
- Add keyboard shortcuts: Ctrl++/- to adjust, Ctrl+0 to reset
- Add font size slider in Settings > Appearance
- Apply font size to Terminal and InputBar via CSS variable
- Persist font size preference between sessions

Closes #19

src/lib/components/Terminal.svelte

@@ -143,7 +143,8 @@
   <div
     bind:this={terminalElement}
     onscroll={handleScroll}
-    class="terminal-content h-[calc(100%-76px)] overflow-y-auto p-4 font-mono text-sm"
+    class="terminal-content h-[calc(100%-76px)] overflow-y-auto p-4 font-mono"
+    style="font-size: var(--terminal-font-size, 14px);"
   >
     {#if lines.length === 0}
       <div class="terminal-waiting italic">