Replace all emojis with custom inline SVG icons across all pages

Each theme gets hand-crafted SVGs matching its visual identity:
- Neon: geometric stroke-based shapes with neon glow drop-shadows
- Invitation: ornamental filled SVGs with calligraphic flourishes,
  gradient balloons, and gold star dividers
- Developer: clean Feather/Octicons-style stroke SVGs with subtle
  party easter eggs (crab with party hat, scattered confetti shapes)
- Index: theme-representative SVG illustrations for each card

No emojis remain in any rendered content.

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

Author: 3 people