Add default video length setting and inherit duration when extending.

Co-authored-by: Cursor <cursoragent@cursor.com>

utils/videoDuration.ts

@@ -0,0 +1,42 @@
+const STORE_KEY = 'aigen-default-video-duration'
+export const VIDEO_DURATION_MIN = 0.5
+export const VIDEO_DURATION_MAX = 120
+export const VIDEO_DURATION_DEFAULT = 5
+
+export function clampVideoDuration(raw: unknown, fallback = VIDEO_DURATION_DEFAULT) {
+  const value = Number(raw)
+  if (!Number.isFinite(value)) return fallback
+  return Math.min(VIDEO_DURATION_MAX, Math.max(VIDEO_DURATION_MIN, Math.round(value * 10) / 10))
+}
+
+export function defaultVideoDurationPref() {
+  if (!import.meta.client) return VIDEO_DURATION_DEFAULT
+  try {
+    return clampVideoDuration(localStorage.getItem(STORE_KEY), VIDEO_DURATION_DEFAULT)
+  } catch {
+    return VIDEO_DURATION_DEFAULT
+  }
+}
+
+export function setDefaultVideoDurationPref(raw: unknown) {
+  const next = clampVideoDuration(raw, VIDEO_DURATION_DEFAULT)
+  if (import.meta.client) {
+    try {
+      localStorage.setItem(STORE_KEY, String(next))
+    } catch { /* ignore */ }
+  }
+  return next
+}
+
+/** Length of one generated part (extension delta), not the stitched family total. */
+export function clipPartDuration(
+  clip: { duration?: number; parentClipId?: string | null },
+  parent?: { duration?: number } | null
+) {
+  const own = Number(clip.duration)
+  const prior = Number(parent?.duration)
+  if (clip.parentClipId && Number.isFinite(own) && Number.isFinite(prior) && own > prior + 0.4) {
+    return clampVideoDuration(own - prior)
+  }
+  return clampVideoDuration(own)
+}