feat(quickshell): add polkit authentication agent

Registers a polkit agent for the logind session and presents its requests in
the hyprchrome panel chrome. PolkitPrompt owns the agent, the layer-shell
surface and focus; PolkitPromptContent is the headlessly renderable visual
core, staged by tests/PolkitPromptHeadless.qml.

Replaces terra's hyprpolkitagent autostart, which had been dead for a while:
the unit was never installed, so the start failed silently and the session
ran with no polkit agent at all.

Verified against a live agent — registration, the PAM conversation, retry
after a rejected attempt, and cancellation. Behaviours found by tracing that
the component now documents:

  * registration is ASYNCHRONOUS, so a Component.onCompleted check reports a
    false failure while a change handler cannot see a total failure at all
    (a failed registration never changes the property) — hence the deadline
  * a flow arrives with isResponseRequired false and an empty prompt, so the
    field is still disabled when the window first becomes visible and the
    re-focus on that transition is load bearing
  * concurrent requests SUPERSEDE rather than queue, orphaning the older one.
    Cancelling it from QML trips "QObject::connect(AuthFlow, PolkitAgentImpl):
    invalid nullptr parameter" upstream and costs the live prompt as well, so
    it is deliberately left alone
  * Identity.id is the raw uid, not unix-user:<name>

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GAq2kKCLazZmrKvkd3akud

dotfiles/quickshell/shell.qml

@@ -15,6 +15,7 @@ import qs.HyprChrome.Widgets
 import qs.HyprChrome.Widgets.Bar.Debug
 import qs.HyprChrome.Widgets.Bar.Host
 import qs.HyprChrome.Widgets.Bar.Vitals
+import qs.HyprChrome.Widgets.Polkit
 
 Scope {
     // Dense multi-monitor status rail; visual core is headlessly renderable.
@@ -34,6 +35,11 @@ Scope {
     CyberDock {}           // 11 — cyberpunk bottom cartridge dock
 
     Notifications {}
+
+    // Polkit authentication agent. Registers for this logind session on
+    // creation, so it replaces hyprpolkitagent rather than coexisting with it
+    // — only one agent may hold a session (see hosts/terra/home/hyprland.nix).
+    PolkitPrompt {}
     VolumeOsd {}
 
     // Host vitals HUD — toggle with SUPER CTRL V.