[timeline][needs-port] 缺少 fade knee 拖拽编辑（DragState.fadeKnee）

[appergb]

上游 DragState.swift 有 case fadeKnee(FadeKneeDrag)，支持在视频片段上直接拖拽 opacity fade 拐点标记编辑淡入淡出。当前 web/src/components/timeline/TimelineContainer.tsx 的 DragState 枚举无此项，用户无法拖拽 fade 拐点。
类别: 剪辑引擎（认领修复）。
来源:与上游一致性审计（10 项之一）。

[appergb]

Claiming this for the current upstream editing parity pass. Scope: inspect upstream fade-knee drag behavior, port the local timeline drag state/hit testing/edit action needed for opacity fade knee editing, add focused tests, then send to separate review.

[appergb]

Continuing the claimed fade-knee parity slice: after drag/hit-test support, I am adding the upstream right-click fade-knee interpolation menu (Linear/Smooth) with focused tests and separate review.

[appergb]

Completed the claimed fade-knee parity continuation for the local worktree.\n\nImplemented:\n- Upstream-style fade-knee right-click menu before normal clip menu routing.\n- Dedicated Linear/Smooth-only fade interpolation menu with checked current edge state.\n- Left edge commits fadeInInterpolation; right edge commits fadeOutInterpolation via setClipProperties.\n- Browser fallback now persists fade interpolation properties for non-Tauri dev/testing.\n- Added focused tests for fadeInterpolationMenuItems and fallback persistence.\n\nSeparate review:\n- Spec compliance reviewer: no issues, APPROVE.\n- TypeScript/code quality reviewer: no issues found; noted eslint is not configured in web.\n\nVerification run after final patch:\n- pnpm -C web test: 17 files / 111 tests passed.\n- pnpm -C web build: passed, with the existing Vite Tauri API dynamic/static import warning.\n- cargo test --workspace: passed.