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

frontend/lib/keyframe-strength.ts

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+/** Newly placed GenSpace stills. Loose enough for Distilled all-guide interpolation. */
+export const DEFAULT_KEYFRAME_STRENGTH = 0.7
+
+/** Omitted/invalid persist and HTTP. Matches the backend KeyframeInput Field default. */
+export const MISSING_KEYFRAME_STRENGTH = 1
+
+/** ArrowUp / ArrowDown and the strength rail step in 5% increments. */
+export const KEYFRAME_STRENGTH_STEP = 0.05
+
+/** Persist, restore, and HTTP all go through this so a bad float cannot 422 or fail project parse. */
+export function clampKeyframeStrength(value: unknown): number {
+  if (typeof value !== 'number' || !Number.isFinite(value)) return MISSING_KEYFRAME_STRENGTH
+  return Math.min(1, Math.max(0, value))
+}
+
+function clampUnit(value: number): number {
+  return Math.min(1, Math.max(0, value))
+}
+
+function roundStrength(value: number): number {
+  return Math.round(clampUnit(value) * 100) / 100
+}
+
+/** Top of the rail is 1 (full lock), bottom is 0 (no lock). */
+export function strengthFromPointer(
+  clientY: number,
+  railRect: { top: number; height: number },
+): number {
+  if (!(railRect.height > 0) || !Number.isFinite(clientY)) return DEFAULT_KEYFRAME_STRENGTH
+  return roundStrength(1 - (clientY - railRect.top) / railRect.height)
+}
+
+export function nudgeKeyframeStrength(strength: number, direction: -1 | 1): number {
+  const current = typeof strength === 'number' && Number.isFinite(strength)
+    ? clampUnit(strength)
+    : DEFAULT_KEYFRAME_STRENGTH
+  return roundStrength(current + direction * KEYFRAME_STRENGTH_STEP)
+}
+
+export function formatKeyframeStrength(strength: number): string {
+  const current = typeof strength === 'number' && Number.isFinite(strength)
+    ? clampUnit(strength)
+    : DEFAULT_KEYFRAME_STRENGTH
+  return `${Math.round(current * 100)}%`
+}