fix: persist application icons across updates

[Sleepython]

Closes #110

Summary

• persist snapshots for application icons referenced by the active configuration under %LOCALAPPDATA%\StarPie\IconCache
• use a SHA-256 key derived from the normalized source path, keeping image data out of config.json
• fall back to the persisted snapshot before Windows Shell can replace a missing executable icon with a generic placeholder
• cover primary actions, nested sub-actions, launch targets, and center actions without requiring config migration

Verification

• dotnet build WinPieGestures/WinPieGestures.csproj -c Release --nologo: 0 warnings, 0 errors
• two-process regression probe: generated a snapshot from a temporary .ico, removed the source file, started a fresh process, and verified the fallback image had the identical PNG SHA-256 hash

This is a non-UI change, so no screenshots are included.