feat(playground): polish UI with consistent sizing, resizable render panel

- Add vertically resizable render panel with drag handle
- Synchronize scroll for input/output preview panes
- Consolidate font sizes into FONT_TINY/SMALL/MEDIUM/LARGE constants
- Remove all desktop/mobile font-size responsive overrides
- Unify icon buttons (nav + settings) into shared iconButton style
- Extract shared PANEL_HEADER_PADDING constants
- Compact header, smaller GitHub icon, dimmer line numbers

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: skovhus