fix: restore visible loading spinner animation

[igboyes]

Summary

• fix the Loader ring rendering as a solid, non-spinning circle because tailwind-merge collapsed the transparent bottom border into the same color-conflict group as the solid sides
• emit per-side border color classes so the transparent bottom survives the merge and the spin animation reads correctly
• narrow Loader's color prop to the shared six-color palette and fix two call sites passing unsupported values that were silently falling back to gray

[linear-code]

VIR-2613

[sourcery-ai]

Hey - I've left some high level feedback:

• Now that Loader.color is a narrowed union without a fallback, it might be worth adding a development-time safeguard (e.g., satisfies Record<LoaderColor, string> or an exhaustive check) so future additions to LoaderColor can’t accidentally be missed in colorToClass.
• Consider extracting the color/class mapping used in the test into a shared constant (or reusing colorToClass) so the test asserts behavior rather than duplicating the specific Tailwind class strings, making future palette changes less brittle.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Now that `Loader.color` is a narrowed union without a fallback, it might be worth adding a development-time safeguard (e.g., `satisfies Record<LoaderColor, string>` or an exhaustive check) so future additions to `LoaderColor` can’t accidentally be missed in `colorToClass`.
- Consider extracting the color/class mapping used in the test into a shared constant (or reusing `colorToClass`) so the test asserts behavior rather than duplicating the specific Tailwind class strings, making future palette changes less brittle.

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[igboyes]

Addressed in 2264bb4: colorToClass is now exported and the test derives its expected classes from it instead of duplicating the Tailwind strings, so palette changes stay in sync automatically.

On the exhaustiveness point: colorToClass is already typed as Record<LoaderColor, string> with a literal object, so TypeScript already errors on any missing or extra key — no further safeguard needed.