bos-settings 0.6.0: Power, Firewall, Users, AUR, Firmware panels

Continues reducing terminal-reliance for graphical system control. Five
new panels, plus three GUI apps shipped for things better served by an
existing dedicated tool than reimplemented in bos-settings
(gnome-disk-utility, gufw, mission-center).

- Power: battery status/health, brightness, charge-limit thresholds where
  the hardware exposes them, TLP profile shown read-only by design (no
  Balanced/Performance switcher — TLP auto-selects by power source, and
  power-profiles-daemon isn't installed because it conflicts with tlp).
- Firewall: ufw enable/disable, add/remove rules, view active rules.
  ufw's own status check requires root (confirmed against the installed
  script — not just changes, reads too), so unlike every other panel this
  one does NOT query state in build(): every view is constructed eagerly
  at app launch, and an unconditional privileged read here would mean a
  polkit prompt on every single bos-settings open. Starts blank with a
  "Status not loaded" placeholder; state loads only on an explicit Refresh
  click, with a guard so refresh's own set_active() doesn't loop back into
  triggering ufw enable/disable.
- Users: add/remove accounts, change passwords. All through pkexec on a
  background thread. Can't remove the account you're currently running as.
- AUR: search via yay. Installing deliberately opens a terminal instead of
  a silent --noconfirm install — yay's interactive PKGBUILD diff review and
  sudo prompt are the actual safety mechanism against a malicious AUR
  package, not a formality worth automating away.
- Firmware: fwupd device list + updates, same stream-output-then-refresh
  pattern as Packages.

packages.x86_64: gnome-disk-utility, gufw, mission-center for disk/firewall/
task-manager GUIs that don't need reinventing inside bos-settings.

src/ui/sidebar.rs

@@ -33,9 +33,12 @@ const fn item_sub(
 pub const SYSTEM_ITEMS: &[SidebarItem] = &[
     item("network", "Network", "network-wireless-symbolic"),
     item_sub("breadcrumbs", "Wi-Fi Profiles", "breadcrumbs", "network-workgroup-symbolic"),
+    item("firewall", "Firewall", "security-high-symbolic"),
     item("sound", "Sound", "audio-volume-high-symbolic"),
+    item("power", "Power", "battery-good-symbolic"),
     item("datetime", "Date & Time", "preferences-system-time-symbolic"),
     item_sub("hyprland", "Display", "hyprland.lua", "video-display-symbolic"),
+    item("users", "Users", "system-users-symbolic"),
 ];
 
 pub const PERSONALIZATION_ITEMS: &[SidebarItem] = &[
@@ -50,6 +53,8 @@ pub const PERSONALIZATION_ITEMS: &[SidebarItem] = &[
 
 pub const MAINTENANCE_ITEMS: &[SidebarItem] = &[
     item("packages", "Packages", "package-x-generic-symbolic"),
+    item("aur", "AUR", "system-search-symbolic"),
+    item("firmware", "Firmware", "software-update-available-symbolic"),
     item("snapshots", "Snapshots", "document-open-recent-symbolic"),
 ];