feat(theme): named theme system v1 (11 themes)

[lngdao]

Summary

• Introduces a named theme system replacing the prior light/dark toggle with 11 curated themes (AMOLED, Catppuccin Latte/Mocha, Tokyo Night, Gruvbox, Nord, One Dark, GitHub Light/Dark, plus defaults).
• Migrates theming to data-theme + data-mode attributes, extracts palettes into per-theme CSS files, and adds a registry with migration helpers for legacy settings.
• Updates the settings UI to a grouped theme Select, ships docs for the architecture + "add a theme" procedure, and adds dev-mode registry consistency assertions.

Test plan

• Verify every theme renders correctly in both light and dark modes on /ds-demo.html
• Confirm legacy settings (old light/dark toggle) migrate cleanly on first launch
• Pre-paint script applies correct theme before first render (no flash)
• AMOLED / GitHub / Nord dark backgrounds have acceptable contrast for bg-strong
• Dev-mode registry assertions do not fire on valid theme definitions

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