[@mantine/core] MantineProvider: Restore dark mode light-color to match v8

[h-harsh]

Closes #8895

In dark mode, --mantine-color-{color}-light-color resolved to shade 0 (near-white), so the light, subtle, and transparent variants lost their color identity and rendered near-white text/icons. In v8 it resolved to shade[primaryShade - 5] (a visible tint of the color) — this is the v8→v9 regression reported in the issue (e.g. an orange subtle ActionIcon rendered white in dark mode).

Root cause

The shared text color was flattened to shade 0 as an unintended side effect of 2cdebb6, whose actual goal was to make the -light backgrounds solid instead of translucent:

- [`--mantine-color-${name}-light-color`]: `var(--mantine-color-${name}-${Math.max(primaryShade - 5, 0)})`,
+ [`--mantine-color-${name}-light-color`]: `var(--mantine-color-${name}-0)`,

--mantine-color-{color}-light-color is the text/icon color shared by the light, subtle, and transparent variants, so this affected all three.

Fix

Restore the v8 value (shade[max(primaryShade - 5, 0)], i.e. shade 3 for the default dark primaryShade of 8) while keeping the solid, no-transparency backgrounds from the original commit:

• subtle / transparent — transparent background, so text now sits on the app background with its color restored (matches v8).
• light — background is darken(shade9, 0.5) (very dark), so shade-3 colored text still contrasts well while regaining color identity.

Changes

• Resolve dark-mode -light-color to shade[max(primaryShade - 5, 0)] in get-css-color-variables.ts.
• Regenerate default-css-variables.css and update the resolver snapshot — the only diff is the 14 default dark -light-color values changing 0 → 3.

Note for maintainers

This changes the appearance of light / subtle / transparent variants in dark mode for everyone, so it's worth a design check. I went with exact v8 parity since that's what the issue asks; happy to target a different shade (e.g. aligning with outline's shade[primaryShade - 4]) if you'd prefer.

Verification

• MantineProvider tests: 69 passing (snapshot updated)
• tsc --noEmit clean, oxlint clean, stylelint clean, formatted
• @mantine/core builds (incl. .d.ts generation)