perf: reduce mobile navigation and rendering cost

[hongding0211]

Summary

This PR reduces mobile rendering pressure and fixes avoidable navigation/cache overhead that made the site feel hot, janky, and slow on phones.

Root causes

• Internal links used non-trailing-slash URLs, causing extra redirects before static pages loaded.
• Static hashed assets, fonts, and images were not given long-lived cache headers.
• Multiple React/Astro islands hydrated on client:load, increasing main-thread work during page load.
• Mobile rendering had expensive effects: large fixed blurred background, backdrop blur, infinite SVG stroke animations, and BlurFade filters.
• Matomo script URL returned 404, adding a failed third-party request.
• The Lottie loading component pulled in a large client dependency and broke SSR during build.

Changes

• Enable trailingSlash: always and update internal links/redirects to slash URLs.
• Add Caddy cache headers for static assets and HTML.
• Defer non-critical hydration from client:load to client:idle / client:visible.
• Reduce mobile/reduced-motion GPU-heavy effects.
• Remove the broken Matomo inline script.
• Replace Lottie loading animation with a lightweight CSS spinner.
• Fix existing TypeScript/build issues in header scroll handling and comment posting.

Validation

• pnpm build passes.
• astro check reports 0 errors.
• Client comments chunk is significantly smaller after removing Lottie.