build: minify CSS/JS at release time instead of tracking .min files

- Add scripts/minify.js using clean-css and terser
- Add "Setup Node.js" and "Minify CSS and JS" steps to both
  .gitea and .github release workflows (runs before packaging)
- Add src/media/{css,js}/*.min.{css,js} to .gitignore
- Remove tracked .min files from the repo (vendor/ untouched)

This eliminates noisy diffs on generated files and ensures the
release ZIP always contains freshly minified assets.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/media/css/editor.min.css

@@ -1 +0,0 @@
-@charset "UTF-8";body{font-size:1rem;font-weight:400;line-height:1.5;color:#22262a;background-color:#fff}h1,h2,h3,h4,h5,h6{margin-top:0;margin-bottom:.5rem;font-weight:700;line-height:1.2}h1{font-size:calc(1.375rem + 1.5vw)}h2{font-size:calc(1.325rem + .9vw)}h3{font-size:calc(1.3rem + .6vw)}h4{font-size:calc(1.275rem + .3vw)}h5{font-size:1.25rem}h6{font-size:1rem}a{text-decoration:none}a:link{color:#224faa}a:hover{color:#424077}p{margin-top:0;margin-bottom:1rem}hr#system-readmore{color:red;border:1px dashed red}span[lang]{padding:2px;border:1px dashed #bbb}span[lang]:after{font-size:smaller;color:red;vertical-align:super;content:attr(lang)}