Create standalone browser port of LTX Athena with authenticated native API transport

frontend/lib/video-constraints.ts

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+// Client-side guard mirroring the LTX API's input constraints, so we reject bad source
+// videos instantly instead of after a slow upload + API round-trip. Dimensions come from
+// the <video> element (videoWidth/videoHeight); the backend/API remain the authority.
+//
+// No aspect-ratio check: the API accepts near-16:9 sizes the docs don't list (e.g.
+// 1920x1024) and ltx-studio doesn't pre-check aspect either, so a strict check
+// false-rejects working videos. The API stays the authority for aspect.
+
+export interface VideoDimensions {
+  width: number
+  height: number
+  duration: number
+}
+
+// API: "Maximum resolution 3840x2160 (4K)", "Minimum frame count 73 (~3s at 24fps)".
+const MAX_LONG_EDGE = 3840
+const MAX_SHORT_EDGE = 2160
+const MIN_DURATION_S = 3
+
+export function validateVideoSource({ width, height, duration }: VideoDimensions): string | null {
+  // Dimensions not known yet (metadata still loading) — don't block prematurely.
+  if (!width || !height) return null
+  const longEdge = Math.max(width, height)
+  const shortEdge = Math.min(width, height)
+  if (longEdge > MAX_LONG_EDGE || shortEdge > MAX_SHORT_EDGE) {
+    return `Video is larger than 4K (${width}×${height}). Maximum supported is 3840×2160.`
+  }
+  if (duration && duration < MIN_DURATION_S) {
+    return `Video is too short (${duration.toFixed(1)}s). It must be at least ${MIN_DURATION_S} seconds.`
+  }
+  return null
+}