feat(settings): Flatpak/Snap sandbox section with copyable permission commands

Tauri parity (the sections did not survive the Slint port): a Settings
section shown only on sandboxed installs holding the one-time permission
commands with copy-to-clipboard buttons. Flatpak: exclusive-DAC D-Bus
reservation override (the user-side fix for #641's exclusive-mode failure
— the Flathub manifest cannot carry the wildcard own-name past its linter,
so the user grants it locally) and NAS/external-drive filesystem access.
Snap: the audio interface connections (qbz-player:alsa/pulseaudio/
pipewire/mpris) plus optional removable-media. New SandboxState global
seeded from qbz_app::diagnostics install_method; 14 msgids across all 8
bundled locales.

km

crates/qbz-ui/ui/state.slint

@@ -5709,6 +5709,19 @@ export global DacWizardActions {
     callback toggle-config(int);     // collapse/expand one DAC's config accordion
 }
 
+// ============================================================================
+// Sandbox (Flatpak/Snap) — Settings section holding the one-time permission
+// commands for sandboxed installs (Tauri parity: Settings > Flatpak / Snap).
+// The section is only shown when install-method is "flatpak" or "snap".
+// ============================================================================
+
+export global SandboxState {
+    // "flatpak" | "snap" | anything else (section hidden). Seeded once at
+    // startup from qbz_app::diagnostics::system_info().install_method.
+    in property <string> install-method: "";
+    callback copy-command(string);   // → crate::share::copy_to_clipboard
+}
+
 // ============================================================================
 // Keyboard shortcuts (hotkeys) — port of the Tauri keybindings system.
 // ============================================================================