feat(assets): integrate UnoCSS for utility classes and presetIcons (#795)

* refactor(assets): integrate UnoCSS for utility classes

- Add UnoCSS with presetWind3 as pre-built CSS (assets/css/unocss.css)
- Add uno.config.ts with theme breakpoints, colors, safelist, and blocklist
- Replace SCSS utility classes with UnoCSS atomic classes:
  - d-none → hidden
  - d-none-desktop → sm:hidden
  - d-none-mobile → max-sm:hidden
  - order-* (safelist)
  - variant-numeric → tabular-nums lining-nums
- Rename .blur to .is-blur to avoid UnoCSS conflict
- Add @unocss-skip comments for SVG path elements to prevent false positives
- Remove redundant _utilities.scss ($orders map) and simplify _common.scss
- Update documentation (CLAUDE.md, CONTRIBUTING.md, copilot-instructions.md)

* refactor(assets): replace 404/offline SCSS with UnoCSS atomic classes

- Replace _404.scss styles with atomic classes on template elements
- Replace _offline.scss styles with atomic classes on template elements
- Keep selectors (id/class) as user customization hooks
- Delete assets/scss/pages/_404.scss and _offline.scss

* refactor(assets): simplify SCSS with UnoCSS atomic classes

- Replace _bilibili.scss with atomic classes (relative, aspect-video, inset-0)
- Replace _version.scss with atomic classes (whitespace-nowrap, align-text-bottom)
- Replace _noscript-warning.scss with atomic classes (bg-danger, fixed, z-200)
- Add semantic z-index shortcuts (z-hide, z-auto, z-base, z-loading, z-sticky, z-fixed)
- Add h1-h6 to UnoCSS blocklist to prevent false positives
- Use Source path for UnoCSS CSS loading to enable Hugo minify

* chore(assets): minor SCSS and template cleanup

* fix(assets): add secondary color to UnoCSS theme config

text-secondary was broken because the secondary color mapping was missing.

* feat(assets): add UnoCSS presetIcons support and z-index shortcuts

- Add presetIcons preset with empty collections (ready for custom icon sets)
- Add @iconify/json and @iconify/utils devDependencies
- Add semantic z-index shortcuts (z-hide, z-auto, z-base, z-loading, z-sticky, z-fixed)
- Add secondary color to UnoCSS theme config

* refactor(assets): replace SVG icons with UnoCSS presetIcons

- Add Lucide and Octicons icon collections to presetIcons config
- Replace arrow-up/down and enter-key SVGs with Lucide icons
- Replace alert icons (info, light-bulb, report, alert, stop) with Octicons
- Delete replaced SVG files from assets/images/icons/
- Keep custom SVGs (csdn, plume, rootme)

* refactor(layouts): redesign 404 and offline pages with UnoCSS

- Replace translate-y-[30vh] with self-stretch flex centering below header
- Remove arbitrary values (text-[3.6rem], text-[1.8rem], etc.)
- Restore @page print styles in _common.scss
- Simplify offline page layout (icon + title + text)
- Fix hardcoded #57606a → text-secondary

.github/copilot-instructions.md

@@ -18,6 +18,15 @@ This document defines the detailed coding standards for the FixIt theme project.
 - Use SCSS variables for colors — no hardcoded values
 - Keep selector nesting shallow
 
+## UnoCSS Guidelines
+
+- Use UnoCSS atomic classes for common utilities (`hidden`, `me-1`, `text-center`, etc.)
+- Theme colors: `bg-primary`, `text-success`, `text-danger`, etc. (mapped via `@theme` in `uno.config.ts`)
+- Responsive: `sm:` (>=681px), `md:` (>=961px), `lg:` (>=1201px), `xl:` (>=1441px). Use `max-sm:` for xs (<681px)
+- Wrap SVG elements with `<!-- @unocss-skip-start -->` / `<!-- @unocss-skip-end -->` to avoid false positives from `d` attribute values
+- Dynamically generated classes (e.g., `order-*`) must be added to `safelist` in `uno.config.ts`
+- Run `pnpm css:build` after modifying `uno.config.ts` or templates that use UnoCSS classes
+
 ## TypeScript Coding Standards
 
 ### Architecture