feat: increase default window size and reduce padding

- Default dimensions: 700x500 (was 600x400)
- Main container padding: 12px (was 16px)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

data/config.example.toml

@@ -34,8 +34,8 @@ tabs = ["app", "cmd", "uuctl"]
 # ═══════════════════════════════════════════════════════════════════════
 
 [appearance]
-width = 600
-height = 400
+width = 700
+height = 500
 font_size = 14
 border_radius = 12