Add user-selectable theme palette

[t3dotgg]

What Changed

• Added client settings for a user theme palette with primary and neutral seed colors.
• Added a theme palette runtime that applies persisted CSS custom properties at boot and on settings updates.
• Updated semantic CSS tokens and General settings UI to generate and preview a palette from selected colors.
• Added focused tests for palette normalization, persistence, and settings schema defaults.

Why

This gives users a predictable way to customize T3 Code colors while keeping the existing light/dark mode and semantic token system intact.

UI Changes

theme_palette_settings_persist.mp4

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

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Note

Add user-selectable theme palette to client settings

• Adds ThemePaletteSettings (primary and neutral hex colors) to ClientSettingsSchema in settings.ts, with validation and defaults.
• Rewires CSS color tokens in index.css to derive from two seed CSS variables (--theme-primary-seed, --theme-neutral-seed), affecting all light/dark mode colors.
• Adds a 'Theme colors' row to the General Settings panel with a color picker, preset swatches, and a live preview component.
• Applies the stored palette at boot (before app mount) via main.tsx and the inline script in index.html to avoid a flash of default colors.
• Behavioral Change: all computed colors (background, card, primary, muted, etc.) now derive from seed colors via color-mix, so existing users will see slightly different shades until they set a custom palette.

^{Macroscope summarized 8a815ad.}

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✨ Finishing Touches

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• Commit unit tests in branch cursor/user-theme-palette-51f1

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