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/tests/PolkitPromptHeadless.qml

@@ -0,0 +1,33 @@
+import QtQuick
+import qs.HyprChrome.Widgets.Polkit
+
+// Offscreen render of the polkit prompt with a failed first attempt and two
+// eligible identities — the state that exercises every optional element at
+// once (picker, pam_error text, rejected-attempt border).
+//
+//   ./tools/quickshell-preview/render.sh \
+//     tests/PolkitPromptHeadless.qml \
+//     .artifacts/quickshell-preview/polkit-prompt.png 560 320
+PolkitPromptContent {
+    width: 520
+
+    message: "Authentication is required to install or remove software"
+    actionId: "org.freedesktop.packagekit.package-install"
+    iconName: "system-software-install"
+    showIcon: false
+
+    identities: [
+        { id: "1000", displayName: "darman", isGroup: false },
+        { id: "0", displayName: "root", isGroup: false }
+    ]
+    selectedIdentity: 0
+
+    responseRequired: true
+    inputPrompt: "Password:"
+    responseVisible: false
+    response: "hunter2"
+
+    supplementaryMessage: "Authentication failure. 2 attempts remaining."
+    supplementaryIsError: true
+    failed: true
+}