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/road.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 576 512"><!-- 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="M573.19 402.67l-139.79-320C428.43 71.29 417.6 64 405.68 64h-97.59l2.45 23.16c.5 4.72-3.21 8.84-7.96 8.84h-29.16c-4.75 0-8.46-4.12-7.96-8.84L267.91 64h-97.59c-11.93 0-22.76 7.29-27.73 18.67L2.8 402.67C-6.45 423.86 8.31 448 30.54 448h196.84l10.31-97.68c.86-8.14 7.72-14.32 15.91-14.32h68.8c8.19 0 15.05 6.18 15.91 14.32L348.62 448h196.84c22.23 0 36.99-24.14 27.73-45.33zM260.4 135.16a8 8 0 0 1 7.96-7.16h39.29c4.09 0 7.53 3.09 7.96 7.16l4.6 43.58c.75 7.09-4.81 13.26-11.93 13.26h-40.54c-7.13 0-12.68-6.17-11.93-13.26l4.59-43.58zM315.64 304h-55.29c-9.5 0-16.91-8.23-15.91-17.68l5.07-48c.86-8.14 7.72-14.32 15.91-14.32h45.15c8.19 0 15.05 6.18 15.91 14.32l5.07 48c1 9.45-6.41 17.68-15.91 17.68z"/></svg>