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

hosts/terra/home/hyprland.nix

@@ -310,7 +310,11 @@ in
           "hyprland.start"
           (lua ''
             function()
-              hl.exec_cmd("systemctl --user start hyprpolkitagent")
+              -- No polkit agent is started here: quickshell registers one
+              -- itself (HyprChrome/Widgets/Polkit), and a session admits only
+              -- one. The hyprpolkitagent line this replaces had been dead for
+              -- a while anyway — the unit was never installed, so the start
+              -- failed silently and the session ran with no agent at all.
               hl.exec_cmd("cosmic-settings-daemon")
               hl.exec_cmd("quickshell")
               hl.exec_cmd("alacritty", { workspace = "special:terminal silent" })