Noctalia v5 launcher freezes ~0.8s on first open per session (NixOS icon lookup)

Symptom: first Mod+D (noctalia msg panel-toggle launcher) after login freezes ~0.4-1s; repeated presses during the freeze toggle the panel open/closed, so it looks buggy. Evidence in ~/.cache/noctalia/noctalia.log: 'surface prepareFrame callback took 795.1ms (..., 788x650 logical)' only on the first open.

Cause: LauncherPanel resolves every app result's icon synchronously on the UI thread on first open (IconResolver::findIcon stats <dir>/<name>.svg/.png across every theme dir). On NixOS, XDG_DATA_DIRS times Adwaita+hicolor gives ~1400 search dirs, and app icons live in hicolor, so every Adwaita dir misses first: ~38k stat() calls for ~45 apps. Measured 160ms even with a warm cache in C++ (paths go through /run/current-system symlinks into the store), 400-1000ms cold. Icon *textures* already load async, so that is not the issue.

Fix idea: resolve app icon names on a worker thread at startup with its own IconResolver (the shared theme state is mutex-protected and thread_local instances are supported), hand the map back via DeferredCall::callLater, and consult it before m_iconResolver.resolve (check the target size and IconResolver::themeGeneration()). Warming only the dentry cache is not enough.

Unrelated: disable the volume-change sound with [audio] enable_volume_sounds = false.