Switch icons from a webfont to self-hosted SVG masks — the webfont was the actual problem

Self-hosting Font Awesome's font files (previous commit) didn't fix it either: every icon in the picker still failed identically. That rules out CDN/network blocking specifically and points to something in the browser blocking custom webfont loading altogether regardless of origin (common with strict anti-fingerprinting protection, e.g. Brave's font blocking or certain privacy extensions — @font-face loading is a well-known fingerprinting vector).

Replaced the whole mechanism: each of the 258 curated icons is now a real downloaded SVG file (static/icons/*.svg, sourced from Font Awesome 5 Free's official SVG package) displayed via CSS mask-image (.icon-svg in style.css) instead of a font glyph. A masked SVG isn't a font resource at all, so it isn't subject to webfont-blocking — and it still colors via the existing "color" style property (background-color:currentColor) and sizes via font-size (1em), so no change to how the widget's other controls work.

The "Icône" widget now stores just the icon's slug (e.g. "trophy") in a dedicated attribute instead of a full CSS class string, rendered by a new special_render (render_icone.py). Updated the add-gallery and the properties-panel picker modal to use the same mask technique for their previews, and element_add.py to validate the slug against the curated list. Removed the now-unused self-hosted Font Awesome CSS/webfont files and the <link> tags — no font dependency left for icons at all.

Verified end to end (gallery renders with real SVG previews, simulated add creates a correctly-attributed element, the SVG file is actually served, the per-element picker reflects the current icon). Full suite green (89).

static/icons/paperclip.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 448 512"><!-- Font Awesome Free 5.15.4 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) --><path d="M43.246 466.142c-58.43-60.289-57.341-157.511 1.386-217.581L254.392 34c44.316-45.332 116.351-45.336 160.671 0 43.89 44.894 43.943 117.329 0 162.276L232.214 383.128c-29.855 30.537-78.633 30.111-107.982-.998-28.275-29.97-27.368-77.473 1.452-106.953l143.743-146.835c6.182-6.314 16.312-6.422 22.626-.241l22.861 22.379c6.315 6.182 6.422 16.312.241 22.626L171.427 319.927c-4.932 5.045-5.236 13.428-.648 18.292 4.372 4.634 11.245 4.711 15.688.165l182.849-186.851c19.613-20.062 19.613-52.725-.011-72.798-19.189-19.627-49.957-19.637-69.154 0L90.39 293.295c-34.763 35.56-35.299 93.12-1.191 128.313 34.01 35.093 88.985 35.137 123.058.286l172.06-175.999c6.177-6.319 16.307-6.433 22.626-.256l22.877 22.364c6.319 6.177 6.434 16.307.256 22.626l-172.06 175.998c-59.576 60.938-155.943 60.216-214.77-.485z"/></svg>