fix: make goose2 respect accent color

[kalvinnchau]

Category: fix
User Impact: Goose2 now applies the selected accent color consistently across themed controls, loading states, and selection surfaces.
Problem: Changing the accent color did not fully affect the Goose2 UI because several theme aliases still resolved to neutral accent tokens. Users could choose a color, but parts of the interface continued to look disconnected from that preference.
Solution: This PR promotes the saved accent color into the core brand tokens, applies matching foreground contrast, and initializes those values before React mounts so the app renders with the correct theme immediately.

File changes

ui/goose2/AGENTS.md
Updates the Goose2 documentation to match the Tailwind CSS version in use and describe the accent token behavior accurately.

ui/goose2/index.html
Adds a pre-mount theme initializer so saved theme, accent, contrast, and density values are applied before React renders.

ui/goose2/src/features/agents/ui/AvatarDropZone.tsx
Uses brand tokens for drag and hover states so avatar interactions reflect the selected accent color.

ui/goose2/src/features/settings/ui/AgentProviderCard.tsx
Updates active provider and loading indicators to use brand styling tied to the saved accent color.

ui/goose2/src/features/settings/ui/ModelProviderPanels.tsx
Uses the brand token for model inventory loading indicators.

ui/goose2/src/features/settings/ui/ModelProviderRow.tsx
Uses the brand token for authentication and inventory sync spinners.

ui/goose2/src/features/settings/ui/ProvidersSettings.tsx
Uses the brand token for provider status loading indicators.

ui/goose2/src/shared/styles/globals.css
Rewires the theme aliases so primary, sidebar, ring, accent, selection, and brand colors inherit from the configured accent color.

ui/goose2/src/shared/theme/ThemeProvider.test.tsx
Adds coverage for accent token application, hex normalization, invalid color fallback, and foreground contrast selection.

ui/goose2/src/shared/theme/ThemeProvider.tsx
Normalizes saved accent colors, computes accessible foreground colors, and applies accent-related CSS variables in one place.

Reproduction Steps

1. Launch Goose2 and open Settings.
2. Change the accent color to a non-default value, such as orange.
3. Open provider settings and model provider panels.
4. Confirm active states, loading indicators, selection color, and branded surfaces use the selected accent color.
5. Reload the app and confirm the accent color is applied immediately without flashing back to the default blue.

accent2.mov

[kalvinnchau]

this said tailwind 3, but the package.json has tailwind 4 https://github.com/aaif-goose/goose/blob/main/ui/goose2/package.json#L124