feat: add swup navigation progress bar for long article loading

[JoeyC-Dev]

Type of change

• Bug fix (a non-breaking change that fixes an issue)
• New feature (a non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Other (please describe):

Checklist

• I have read the CONTRIBUTING document.
• I have checked to ensure that this Pull Request is not for personal changes.
• I have performed a self-review of my own code.
• My changes generate no new warnings.

Related Issue

Resolve: #637

• Addresses the perceived freeze (4-5s) when navigating to long articles via swup, where the DOM replacement blocks the main thread with no visual indication of progress.

Changes

• Add progress bar HTML element, CSS styles (loading animation, finishing, done states), and swup hook integration (visit:start, visit:end). The bar uses a CSS keyframe animation that gradually fills from 0% to 95% over 8 seconds, then snaps to 100% when the navigation completes.
• All tunable values are extracted into CSS custom properties (--progress-height, --progress-duration, --progress-hue-start, --progress-hue-end, --progress-glow) for easy customization.

How To Test

Use 3G mode to check while switching pages.

Screenshots (if applicable)

[vercel]

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