Change keyboard hotkeys from arrow keys to WASD

## Summary - Replace arrow key navigation with WASD keys for subtitle control - A: previous subtitle (was ArrowLeft) - D: next subtitle (was ArrowRight) - S: replay current subtitle (was ArrowDown) ## Test plan - [ ] Build the extension with `pnpm build` - [ ] Load extension in browser and test on a streaming service - [ ] Verify A key moves to previous subtitle - [ ] Verify D key moves to next subtitle - [ ] Verify S key replays current subtitle - [ ] Verify Alt+A and Alt+D work for force navigation 🤖 Generated with [Claude Code](https://claude.com/claude-code)