fix: improve Stories navbar contrast

[saurabhhhcodes]

Summary

Improve navbar contrast on the Stories route in light theme by applying a route-scoped glass background, border, shadow, and readable navigation colors.

Related Issue

Fixes #554

Changes

• Add a Stories-only header class when the /stories route is active.
• Give the light-theme Stories header a distinct dark glass surface and readable nav text.
• Preserve existing header styling on all other routes.

Validation

• git diff --check
• npm run build

Checklist

• Scoped the change to the reported route.
• Preserved shared header behavior elsewhere.
• Ran the production build.

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