docs(themes): optimize for visual learners with diagrams and tables

Major improvements to themes.md documentation:

Quick Reference (Critical Info First):
- Added comparison table of all 5 themes at the top
- Quick start code example immediately visible
- Use case mapping for each theme

Visual Diagrams Added:
- Theme structure tree diagram
- Theme lifecycle flow diagram
- Theme stack visualization with ASCII art
- Visual flow for withTheme() auto-restore
- Output preview examples with colored symbols

Better Organization:
- Clear sections with horizontal rules
- Emoji section markers for easy scanning
- Comparison tables for theme attributes
- Named colors reference table
- Color types comparison table
- Do's and Don'ts with clear markers
- Troubleshooting in Q&A format

Enhanced Content:
- Added "When to use Ultra" guidance
- Migration guide with before/after examples
- Common patterns section with 3 recipes
- Best practices section with actionable tips
- More code examples with inline comments
- TypeScript type definitions reference

For Visual Learners:
- ASCII diagrams for concepts
- Tables for quick scanning
- Preview outputs with comments
- Step-by-step flows
- Consistent formatting patterns
- Clear visual hierarchy

Result: 697 lines (vs 524) - more content, better structured,
easier to scan, critical info first, optimized for learning.

Author: jdalton