add custom integer playback speed to live view video player

[ehfeng]

Summary

Adds a numeric input on the live-view video controls bar so users can set any positive integer playback speed (1x, 2x, 3x, 5x, 10x, …), removing the implicit 2x cap that came from the native browser controls.

• new `playbackRate` state + `setPlaybackRate` mutation in `store/video.ts`
• `video.vue` watches the store value and applies it to the underlying `
• `controls.vue` renders a small number input (min=1, step=1) next to the play/pause button

Test plan

• Built locally with `npx vue-cli-service build` — clean
• Open a live view, type 3 (or higher) in the speed input and confirm the stream speeds up
• Confirm non-integer / sub-1 input is clamped to a valid integer
• Confirm typing in the input does not send keystrokes to the remote (`@keydown.stop`)