feat(arch-layer,scripts): linux-gated desktop configs + package install scripts

niri (exact_config.d, scripts), noctalia (plugins excluded — runtime
state with embedded token), fontconfig, mimeapps.list,
niri-dynamic-workspaces, systemd user unit files (enable-state not
managed; only nirinit enabled by run_once), niri-launch vendored from
/usr/bin (unowned hand-written script, shellcheck warnings pre-existing).

Scripts: packages (pacman --needed + paru, brew on darwin, root-safe,
keyed on packages.yaml hash), gh extensions (idempotent), user services
(linux), post (tpm clone, bat cache, fc-cache). All shellcheck-clean.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

dot_config/niri/exact_config.d/40-environment.kdl

@@ -0,0 +1,22 @@
+// ============================================================================
+// ENVIRONMENT
+// ============================================================================
+
+environment {
+    XDG_CURRENT_DESKTOP "niri"
+    XDG_MENU_PREFIX "plasma-"  // Required for Dolphin/KDE app file associations
+    QT_QPA_PLATFORM "wayland"
+    ELECTRON_OZONE_PLATFORM_HINT "auto"
+    QT_LOGGING_RULES "quickshell.dbus.properties=false"
+    
+    // Qt theming: kde platform reads colors from kdeglobals (requires plasma-integration pkg)
+    // The installer falls back to qt6ct only if plasma-integration is not installed
+    QT_QPA_PLATFORMTHEME "kde"
+    QT_STYLE_OVERRIDE "Darkly"
+    
+    // Quickshell Python virtual environment
+    INIR_VENV "$HOME/.local/state/quickshell/.venv"
+    ILLOGICAL_IMPULSE_VIRTUAL_ENV "$HOME/.local/state/quickshell/.venv"
+}
+
+