🎨 style(app): dramatic visual atmosphere layer — depth, gradients, elevated panels

App-level CSS overrides for production-grade look:
- Sidebar: gradient background with depth border
- Message area: subtle gradient background
- Prompt dock: elevated floating shadow with 16px radius
- Code/bash blocks: terminal-grade 10px radius + inset shadow
- File write/edit tools: card-style border treatment
- User messages: chat bubble with 18px radius + interactive color
- Titlebar: subtle bottom border + shadow
- Dialogs: premium multi-layer shadow + 12px radius
- Popovers/menus: elevated shadow treatment
- Numbered lists: custom counter with blue numbers + font-weight 600
- Thinking state: contained in bordered pill
- Collapsible triggers: hover background feedback
- Permission prompts: warning border + shadow elevation
- Scrollbar: thin 6px styled scrollbar on message area
- Empty state: subtle radial gradient atmosphere

Author: praxstack