feat(quickshell): give the prompt its own panel, ESC handling and placement

PolkitPanel replaces BarPanel as the dialog's chrome. Deliberately not a
subclass or a fork: most of BarPanel is density machinery — summary slot,
animated height, state pair, transitions — that a modal never uses, and
inheriting it would tie the dialog's look to a component whose real job is the
rail, so every restyle here would have to be justified against the panels up
there. It keeps the shell's silhouette (cut corners with detached accent caps,
the accent rules, the header strip) and drops the rail's tick decoration.

ESC closes the rail. The bar had no keyboard focus at all, so this adds it,
gated by the shell rather than left to the compositor to arbitrate between two
exclusive surfaces — that resolves by stacking and would invert silently the
day the layers change:

    grabsKeyboard: shell.expanded && !polkit.prompting

so ESC dismisses the prompt while one is open and closes the rail afterwards.
Verified by instrumenting the handoff: expanded -> true, prompt open -> false,
prompt dismissed -> true, collapsed -> false. Note the rail now takes EXCLUSIVE
keyboard focus while expanded, which is the cost of answering a keypress the
user has not aimed at anything.

The dialog sits a third of the way down rather than centred, panel centre on
the third so it grows symmetrically as the message wraps, floored at a margin
so a tall prompt on a short output cannot be pushed off the top.

Also carries the backdrop tuning: dim 0.75 -> 0.65, gridOpacity 0.15 -> 0.10,
crossOpacity 0.45 -> 0.15, now that the scrim is used by the prompt as well as
the rail.

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

dotfiles/quickshell/HyprChrome/Widgets/ChromeBackdrop.qml

@@ -32,7 +32,7 @@ PanelWindow {
     // of a pair with the bar's, so the shell derives both.
     property int wlrLayer: WlrLayer.Top
 
-    property real dim: 0.75
+    property real dim: 0.65
     property int gridSpacing: 60
 
     // Height of the collapsed rail, including its margins — the band the scrim
@@ -84,14 +84,14 @@ PanelWindow {
     // The dense bar drew this grid at 0.018 against its own near-black panel.
     // Over a 55% scrim on top of lit windows that is invisible, so it is a
     // knob rather than a constant.
-    property real gridOpacity: 0.15
+    property real gridOpacity: 0.10
 
     // The accent with its saturation pulled back: warm enough to read as part
     // of the palette, not so loud that a full-screen grid competes with the
     // bar. Derived rather than a literal so it tracks a palette change.
     // Registration crosses sit on every other intersection of the grid.
     property color crossColor: Theme.muted
-    property real crossOpacity: 0.45
+    property real crossOpacity: 0.15
     property int crossSize: 20
 
     // Thickness in STEPS, not pixels: 1 -> 1px, 2 -> 3px, 3 -> 5px. Only odd