fix: prevent keyboard relative speed baseline reset after tab idle

[MuizNadeem]

Fixes #1494

Summary

This PR fixes a regression where keyboard relative speed shortcuts (s / d) could apply from a 1.0x baseline after tab backgrounding, instead of from the active/effective remembered speed.

Changes

• Use remembered non-1.0 speed as baseline for the first relative step when:
  • current playback rate is effectively 1.0, and
  • delta is a typical shortcut step.
• Treat lifecycle speed restoration as init-like (source: 'init') in media event restore path.
• Do not persist lastSpeed for source: 'init' updates.

Why

Lifecycle/init reconciliation should restore playback without mutating user-authoritative remembered speed. If lifecycle restore writes lastSpeed, first keyboard relative adjustment can compute from an incorrect 1.0 baseline after idle/background transitions.

Test Plan

• Added unit test: relative increase uses remembered baseline after idle reset to 1.0.
• Added unit test: play-event restoration does not overwrite lastSpeed.
• Manual:
  • Enable rememberSpeed
  • Set speed to 1.8x
  • Background tab and return
  • Press d/s
  • Verify behavior is 1.9x / 1.7x (not 1.1x / 0.9x)

[MuizNadeem]

Hi @igrigorik, could you please review this PR?