feat: add gray40 and gray50 variables

[huyenltnguyen]

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Currently we default to gray45 for muted/disabled text. However, it does not provide sufficient contrast against our background colors. The only case where this is acceptable is for disabled elements, but there are recent use cases where the color is used for non-disabled content as well.

This PR introduces two new gray variables for muted text:

• Light theme will use gray50
• Dark theme will use gray40

These colors meet the contrast ratio requirements against primary, secondary, and tertiary background colors. Quaternary backgrounds are not included, as I haven't seen muted/disabled text used on them.

Contrast ratio

Light theme

	primary bg (#ffffff)	secondary bg (#f5f6f7)	tertiary bg (#dfdfe2)
gray45 (#858591)	❌ 3.64:1	❌ 3.36:1	❌ 2.74:1
gray50 (#626262)	✅ 6.09:1	✅ 5.63:1	✅ 4.58:1

Dark theme

	primary bg (#0a0a23)	secondary bg (#1b1b32)	tertiary bg (#2a2a40)
gray45 (#858591)	✅ 5.33:1	✅ 4.61:1	❌ 3.83:1
gray40 (#939393)	✅ 6.32:1	✅ 5.47:1	✅ 4.54:1