Add buttons, menus and the rest of M3 Expressive's actions

<x-button> (label buttons, icon buttons and toggles in five sizes, with
filled, tonal, outlined, elevated and text variants in any colour role),
<x-tooltip>, <x-menu> with items, groups and separators, <x-button-group>,
<x-group> as a connected button group, <x-split-button>, <x-fab>,
<x-fab-menu> and <x-loading>. Sizes, colours and shapes come from
androidx Compose Material 3's tokens; the loading indicator ports its
Morph into SVG + SMIL.

Menus follow WAI-ARIA's menu button pattern on popovers placed by CSS
anchor positioning. Browser tests run in Chromium, Firefox and WebKit.
The showcase fetches the icon names on demand: inlined, they tripped
Pest's test server into HTTP 431s under Firefox.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01V9NnLxnPp8vaaurb3Z1MFy

package.json

@@ -6,7 +6,8 @@
         "build": "vite build",
         "dev": "vite",
         "build:scheme": "esbuild bin/scheme.mjs --bundle --platform=node --format=esm --target=node20 --minify --legal-comments=eof --outfile=resources/node/scheme.mjs",
-        "build:shapes": "node bin/shapes.mjs"
+        "build:shapes": "node bin/shapes.mjs",
+        "build:loading": "node bin/loading-indicator.mjs"
     },
     "devDependencies": {
         "@material/material-color-utilities": "^0.4.0",