🎨 feat(ui): complete visual overhaul matching reference design

24 visual features implemented via CSS:
1. Grain texture overlay (SVG noise at 2.5% opacity on body)
2. Glass morphism user bubbles (backdrop-filter blur, shadow, rounded-tr-none)
3. Assistant avatar label ('⬡ OPENCODE AI' uppercase, purple, indented content)
4. User 'You' label (cyan, right-aligned)
5. Thinking block (pulsing cyan dot, card container, cursor pointer)
6. Reasoning part (bordered card with mono font, border-top separator)
7. Code blocks (12px radius, dark inset background)
8. Copy button (always visible, hover → primary, copied → green)
9. Glass prompt dock (backdrop-blur, elevated shadow)
10. Send button glow (20px purple shadow)
11. Sidebar (dark background-strong)
12. Titlebar (backdrop-blur, border)
13. Dialog (premium multi-layer shadow)
14. Popovers/menus (backdrop-blur, elevated)
15. Settings (12px radius)
16. Active tab indicator (blue bottom border)
17. Numbered lists (blue counters)
18. Markdown headings (theme --markdown-heading color)
19. Markdown bold/italic/links/code (theme colors)
20. File write/edit (card border treatment)
21. Bash output (terminal card, 200px max-height)
22. Collapsible hover feedback
23. Permission prompts (warning border + shadow)
24. Thin styled scrollbar + empty state gradient

Author: praxstack