feat: playback speed control

[EL-File4138]

Changes

• Adds one shared playback-rate control for audio-only playback and synced audio/video playback.
• Adds transport-rate commands plus default hotkeys for decrease, increase, and reset.
• Adds a playback-rate submenu in Video and a bottom-right editable dropdown in the audio pane.
• Persists the selected rate and supports arbitrary typed values in the supported 0.1x to 3.0x range.

Implementation

• Source time remains the single authoritative timeline for subtitle timing, waveform display, direct seeking, and frame lookup.
• Speed change is implemented as a multiplier on the current playback pipeline:
  • Video: Apply the multiplier on measured source time change, advancing playback transport, resolving the displayed frame.
  • Audio: Spin off a playback-scoped resampling provider layered above the existing audio backends, apply the multiplier here.
• During playback, controller code converts between source-domain samples/time and playback-domain samples so UI reporting stays source-accurate.
• Changing playback rate while transport is active rebuilds playback coherently and resumes from the current source position.

Consequences

• Precise editing behavior is preserved while paused: seeking, stepping, timing edits, and frame lookup stay on the original source timeline.
• Audio playback speed changes use resampling, so pitch changes with speed.
• Audio playback speed is also applied when playing segments (using commands like audio/play/current)
• Video playback at rates above 1.0x can skip displayed frames when source-time advancement outpaces the UI timer cadence; this is an intentional transport-level compromise and keeps the implementation aligned with the project’s existing playback model.
• Either control honors playback speed individually without cross-dependence, while staying in sync when both are used;
• Arbitrary playback-rate input is supported and clamped safely, while step commands remain restricted to the intended coarse preset set.