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/warehouse.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 640 512"><!-- 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="M504 352H136.4c-4.4 0-8 3.6-8 8l-.1 48c0 4.4 3.6 8 8 8H504c4.4 0 8-3.6 8-8v-48c0-4.4-3.6-8-8-8zm0 96H136.1c-4.4 0-8 3.6-8 8l-.1 48c0 4.4 3.6 8 8 8h368c4.4 0 8-3.6 8-8v-48c0-4.4-3.6-8-8-8zm0-192H136.6c-4.4 0-8 3.6-8 8l-.1 48c0 4.4 3.6 8 8 8H504c4.4 0 8-3.6 8-8v-48c0-4.4-3.6-8-8-8zm106.5-139L338.4 3.7a48.15 48.15 0 0 0-36.9 0L29.5 117C11.7 124.5 0 141.9 0 161.3V504c0 4.4 3.6 8 8 8h80c4.4 0 8-3.6 8-8V256c0-17.6 14.6-32 32.6-32h382.8c18 0 32.6 14.4 32.6 32v248c0 4.4 3.6 8 8 8h80c4.4 0 8-3.6 8-8V161.3c0-19.4-11.7-36.8-29.5-44.3z"/></svg>