chore(widgets): Add PostCSS build pipeline for icon management

[chrisgervang]

Closes #10512

Background

The widget stylesheet embeds ~15 SVG icons as inline data URLs with full SVG markup manually copy-pasted into CSS. This makes icons difficult to audit, update, and diff.

Change List

• Store icons as separate .svg files in src/icons/ — one file per icon, trivially auditable and diffable
• Replace cp build with PostCSS pipeline using postcss-inline-svg to resolve svg-load() directives into inline data URIs at build time
• Add autoprefixer as a safety net for future vendor prefix needs (adds -webkit-mask-image back in the output for broader compat)
• Source CSS uses readable svg-load("icons/fullscreen.svg") syntax instead of multi-line data URLs

Prior Art

• MapLibre GL JS uses exactly this setup: postcss + postcss-inline-svg + autoprefixer, with SVGs stored in src/css/svg/

Source CSS before → after

/* Before: inline data URL */
mask-image: var(--icon-fullscreen-enter, url('data:image/svg+xml,<svg xmlns=...>...</svg>'));

/* After: svg-load directive (resolved at build time) */
mask-image: var(--icon-fullscreen-enter, svg-load("icons/fullscreen.svg"));

Stack

This PR is stacked on #10513 (icon cleanup PR).

---

Note

Low Risk
Build and asset-pipeline changes only; runtime CSS behavior should match prior inlined icons, with no security or data-handling impact.

Overview
Replaces inline SVG data URLs in the widget stylesheet with svg-load() references to files under src/icons/, and builds the shipped CSS through PostCSS instead of copying source CSS verbatim.

The @deck.gl/widgets build script now runs postcss with postcss-inline-svg (icons → inline data URIs at build time) and autoprefixer (e.g. -webkit-mask-image). deck.gl and pydeck dev hot-reload now read modules/widgets/dist/stylesheet.css so they pick up the processed output. Root devDependencies add postcss, postcss-cli, postcss-inline-svg, and autoprefixer.

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[coveralls]

coverage: 83.748% (-0.01%) from 83.759% — chr/postcss-pipeline-v1 into master

[chrisgervang]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Merged master into this branch and resolved the yarn.lock conflict by regenerating it with yarn install. The branch is now up to date with master (commit ceb5ba4).