fix(ui): replace misleading permission tooltip on segment override variations

[nielskaspers]

Summary

• Fixes misleading "Create Feature permission" tooltip shown on read-only variation value inputs in segment overrides
• When variations are read-only due to being in a segment override context, the tooltip now shows: "Variation values are defined at the feature level and cannot be changed per segment."
• The original permission tooltip is preserved for cases where the user genuinely lacks Create Feature permission

Issue

Fixes #6982

Changes

• frontend/web/components/mv/VariationValueInput.tsx: When readOnly is true and canCreateFeature is falsy, show an informative tooltip instead of the permission-related one

Testing

1. Create a multivariate feature with variations
2. Go to Segment Overrides tab
3. Add or view a segment override
4. Hover over a variation value input
5. Verify the tooltip now says "Variation values are defined at the feature level and cannot be changed per segment."
6. Verify that non-segment-override variation editing still shows the correct permission tooltip when the user lacks Create Feature permission

[claude]

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