Add focused app volume hotkeys

[Alb11747]

Summary

• add user-configurable hotkeys to raise, lower, or toggle mute for the focused app
• expose the new shortcuts in EarTrumpet's shortcut settings
• share foreground-app lookup with the Actions add-on instead of maintaining two implementations

Focused-app resolution

• resolve regular Win32 and hosted ApplicationFrameWindow apps
• target an active descendant audio app when exactly one distinct descendant owns an active session, which covers injected overlays
• fall back to the actual foreground app when no active descendant exists or the result is ambiguous

Volume behavior

• use a 2 percentage-point step in linear mode and a 0.5 dB step in logarithmic mode
• clamp volume to the active mode's valid range and recover from non-finite session values
• toggle mute consistently across every matching session for the focused app

Validation

• Release builds pass for x64, x86, and ARM64

Scope

• volume toast notifications are intentionally not included in this pull request

[CLAassistant]

All committers have signed the CLA.

[Loukious]

You may want to add a mute/unmute foreground app while at it. I think that would be the most important feature for gamers.