feat(frontend): bundle the icons so the app works offline

Every <wa-icon> was fetched from ka-f.fontawesome.com at runtime —
confirmed from `performance.getEntriesByType('resource')`, 36 requests
— so offline the app had no icons at all. `setBasePath()` does not
affect the icon resolver; only the component autoloader reads it.
Overriding Web Awesome's `default` icon library fixes all 165 call
sites without changing one of them. Cross-origin requests at startup:
22 -> 0.

Three things about it are load-bearing. The set is Font Awesome Free
(CC BY 4.0, vendored with its licence by `scripts/fetch-icons.mjs`)
because the kit CDN serves Pro, which cannot be redistributed. The
names are a committed list rather than anything derived, because
twenty call sites compute their icon name from state and no static
pass can enumerate them. And a name that is not bundled is reported at
runtime to `window.__yjIconMisses` and drawn as a fallback, since a
missing icon used to be impossible — the CDN having had everything.

frontend/src/icons/index.ts

@@ -0,0 +1,103 @@
+/**
+ * Bundled icons.
+ *
+ * Web Awesome's default icon library resolves every `<wa-icon>` to
+ * `https://ka-f.fontawesome.com/releases/v7.1.0/svgs/<style>/<name>.svg`
+ * and fetches it at runtime.  `setBasePath()` does not change that —
+ * it is only read by the component autoloader — so a desktop music
+ * player offline, on a captive portal or behind a firewall rendered no
+ * icons at all (audit H-4, perf.M9), and a cold start waited on
+ * fontawesome.com for up to 36 cross-origin requests.
+ *
+ * Overriding the library named `default` replaces that resolver for
+ * every existing call site at once: no component changes, no icon
+ * renamed, nothing to remember at the next one.
+ *
+ * The SVGs are emitted as assets rather than inlined into the JS.
+ * Inlining 64 files would put ~270 kB of markup into a bundle that is
+ * already the subject of perf.M10, and would make every icon part of
+ * the startup parse; as assets they are served by the app's own asset
+ * handler, cached by the browser, and fetched only when first used.
+ */
+
+import { registerIconLibrary } from '@awesome.me/webawesome/dist/webawesome.js';
+
+/**
+ * name -> emitted asset URL, built at compile time.
+ *
+ * `eager` matters: a lazy glob would make the resolver async, and Web
+ * Awesome's resolver is synchronous.
+ */
+const FILES = import.meta.glob<string>(
+    '../assets/icons/fa/**/*.svg',
+    { eager: true, query: '?url', import: 'default' },
+);
+
+/** `solid/house` and `house` both resolve; call sites use the latter. */
+const BY_NAME = new Map<string, string>();
+
+for (const [path, url] of Object.entries(FILES)) {
+    const m = /\/fa\/([^/]+)\/([^/]+)\.svg$/.exec(path);
+
+    if (!m?.[1] || !m[2]) continue;
+
+    const family = m[1];
+    const name = m[2];
+
+    BY_NAME.set(`${family}/${name}`, url);
+
+    // `solid` is Web Awesome's default family, so a bare name means the
+    // solid one.  A regular-family icon that shares its name (heart)
+    // must not overwrite it.
+    if (family === 'solid' || !BY_NAME.has(name)) {
+        BY_NAME.set(name, url);
+    }
+}
+
+/**
+ * Names asked for that are not bundled.
+ *
+ * A missing icon used to be invisible — the CDN had everything, so
+ * nothing ever failed.  Now it has to be *findable*, because twenty
+ * call sites compute their name from state and no static check can
+ * enumerate them (see `src/icons/names.txt`).  Recording the miss and
+ * rendering a placeholder makes `frontend/scripts/icon-sweep.mjs` able
+ * to report them, and makes a real one look wrong rather than absent.
+ */
+const misses = new Set<string>();
+
+declare global {
+    interface Window {
+        __yjIconMisses?: string[];
+    }
+}
+
+function report(name: string): void {
+    if (misses.has(name)) return;
+
+    misses.add(name);
+    window.__yjIconMisses = [...misses];
+    console.error(
+        `icon '${name}' is not bundled; add it to src/icons/names.txt ` +
+        'and run: node frontend/scripts/fetch-icons.mjs',
+    );
+}
+
+const FALLBACK = BY_NAME.get('circle-question') ?? '';
+
+export function registerBundledIcons(): void {
+    registerIconLibrary('default', {
+        resolver: (name: string) => {
+            const url = BY_NAME.get(name);
+
+            if (url) return url;
+
+            report(name);
+
+            return FALLBACK;
+        },
+    });
+}
+
+/** Exported for the test that asserts every listed name resolves. */
+export const bundledIconNames = (): string[] => [...BY_NAME.keys()];