Make Panama settings one shared source of truth

Panama had grown into three configuration surfaces that only agreed because
they had been typed to agree: looks.lua hardcoded values, DesktopPreferences
independently defaulted the same values, and SystemSettings replayed them at
startup. Nothing kept them in sync, and the Lua side read no shared state at
all. This lands the first three stages of docs/superpowers/plans/2026-08-17-panama-cohesion.md.

Fix silently failing Hyprland writes. On a Lua-configured Hyprland, hyprctl
keyword refuses the write, prints the refusal to stdout, and still exits 0, so
the HDR, VRR, and direct-scanout toggles persisted their value and reported
success while the compositor never changed. Writes now go through hyprctl eval,
which has the same hazard on syntax and runtime errors, so success is defined
as reading the value back and finding it equal. The existing contract passed
throughout the outage because it re-applied the values already in place; the
new one flips each value to something it does not hold.

Derive preferences from a schema. Every setting used to be restated four times
-- a property alias, a JSON adapter property, a change handler, and a line in
reset -- where omitting any one failed silently. PreferenceSchema.qml is now
the single source, and persistence, validation, reset, and the Hyprland mapping
all derive from it. Unknown keys on disk survive a write so a rollback does not
discard a newer build's settings, and a corrupt file falls back to shipped
defaults. The store moved to ~/.config/panama/settings.json, migrating from the
old state directory without deleting it.

Share that file with Hyprland. prefs.lua reads it at config time with every
shipped literal kept as the fallback, so the config still stands alone. The Lua
is the default, the JSON is the truth, and Settings is the editor. The
compositor-adjustable surface goes from 3 keys to 23.

Also fixes two test-hygiene bugs found by running the suite end to end for the
first time: settings-pages-contract could see the window settings-window-contract
leaves behind, and the new write contract was persisting its deliberately-wrong
values into the user's real store.

Claude-Session: https://claude.ai/code/session_01BRvzt4H8XXLPVH5MyYdk9L

config/dot/hypr/input.lua

@@ -5,28 +5,30 @@
 -- acceleration changes. The goal is that muscle memory transfers untouched.
 -- ─────────────────────────────────────────────────────────────────────────────
 
+local prefs = require("prefs")
+
 hl.config({
     input = {
-        kb_layout  = "us",
+        kb_layout  = prefs.get("keyboardLayout", "us"),
         kb_variant = "",
         kb_model   = "",
         kb_options = "",
         kb_rules   = "",
 
-        numlock_by_default = true,
+        numlock_by_default = prefs.get("numlockByDefault", true),
 
         -- GNOME's defaults are 500ms delay / 33Hz repeat.
-        repeat_delay = 500,
-        repeat_rate  = 33,
+        repeat_delay = prefs.get("keyRepeatDelay", 500),
+        repeat_rate  = prefs.get("keyRepeatRate", 33),
 
         -- 1 = click to focus. GNOME's behaviour; NOT sloppy focus.
-        follow_mouse = 1,
+        follow_mouse = prefs.getInt("followMouse", 1),
 
         -- Don't refocus on mouse move alone -- only on click.
         mouse_refocus = false,
 
         -- Flat pointer response, no acceleration. Matters for gaming.
-        sensitivity   = 0,
+        sensitivity   = prefs.get("pointerSensitivity", 0),
         accel_profile = "flat",
 
         -- Clicking a floating window raises and focuses it.