Cancel stale playback probes and verify Linux QSV support

public/js/components/PlaybackProbe.js

@@ -0,0 +1,28 @@
+/** Metadata probes belong to the active playback, and share an ordered lease across replacements. */
+class PlaybackProbe {
+    constructor(fetchImpl = fetch) {
+        this.fetch = fetchImpl;
+        this.sequence = 0;
+        this.leaseId = Array.from(globalThis.crypto.getRandomValues(new Uint8Array(16)),
+            byte => byte.toString(16).padStart(2, '0')).join('');
+    }
+    async read(url, { program = null, userAgent } = {}) {
+        this.cancel();
+        const controller = this.controller = new AbortController();
+        const query = new URLSearchParams({ url, probeLeaseId: this.leaseId, requestSequence: String(++this.sequence) });
+        if (program != null) query.set('program', program);
+        if (userAgent !== undefined) query.set('ua', userAgent);
+        try {
+            const response = await this.fetch('/api/probe?' + query, { signal: controller.signal, credentials: 'same-origin' });
+            const info = await response.json();
+            if (controller.signal.aborted || this.controller !== controller || response.status === 409) {
+                throw Object.assign(new Error('Playback probe was superseded'), { name: 'AbortError' });
+            }
+            if (response.ok === false) throw new Error(info.error || 'Unable to probe stream');
+            return info;
+        } finally { if (this.controller === controller) this.controller = null; }
+    }
+    cancel() { this.controller?.abort(); this.controller = null; }
+}
+if (typeof window !== 'undefined') window.PlaybackProbe = PlaybackProbe;
+if (typeof module !== 'undefined') module.exports = PlaybackProbe;