feat(ui): Display tab + settings.json persistence — first non-keybind tab lands

Phase L.0 (cont.) — first concrete tab on the new Settings shell, in
the Easy-wins build order agreed in the brainstorm
(Display → Audio → Gameplay → Chat → Character).

DisplaySettings (immutable record): Resolution / Fullscreen / VSync /
FieldOfView (30-120°) / Gamma (0.5-2.0) / ShowFps. Six common 16:9
resolutions in the dropdown. Defaults: 1920×1080, windowed, vsync on,
75° FOV, gamma 1.0, FPS off — matches the brainstorm UX agreement.

SettingsStore: JSON persistence at %LOCALAPPDATA%\acdream\settings.json
(coexists with keybinds.json — own load/save path stays put, no
migration needed). LoadDisplay falls back per-field when keys are
missing (partial-file tolerant) and falls back to defaults when the
file is corrupt or the JSON is unparseable. SaveDisplay round-trips
preserved — unknown top-level keys (e.g. an `audio` section written
by a future client) are kept on save so older builds don't silently
drop newer-tab data.

SettingsVM gains a parallel display-state machine: persistedDisplay +
draftDisplay, SetDisplay mutator, HasUnsavedChanges checks both
keybinds and display deltas, Save/Cancel/ResetAll cover both
atomically from the user's POV (one Save commits everything, one
Cancel reverts everything). Constructor signature extends with two
new params; existing keybinds-only callers updated.

SettingsPanel.RenderDisplayTab replaces the L.0-shell placeholder —
Combo for resolution, Checkboxes for fullscreen/vsync/show-fps,
SliderFloat for FOV + gamma. Live-preview note in the panel body
matches the agreed UX: FOV + gamma update visibly while the user
drags; resolution / fullscreen / vsync apply on Save (live preview
would be too jarring).

GameWindow wires SettingsStore into the existing SettingsVM construct
site — load on startup, save on each tab Save. Errors print to
console and don't crash the panel.

19 new tests:
 · DisplaySettings record (4) — defaults pinned, value equality, with-
   expressions, AvailableResolutions sorted ascending
 · SettingsStore (6) — round trip, missing-file → defaults, corrupt-
   file → defaults, partial-file → per-field fallback, unknown-key
   preservation, DefaultPath shape
 · SettingsVM display (6) — initial draft tracks persisted, SetDisplay
   marks dirty, Save invokes display callback, Cancel reverts,
   ResetAllToDefaults covers display, Save-then-Cancel is no-op
 · SettingsPanel display tab (3) — widgets render only when active,
   resolution combo uses AvailableResolutions, no Combo emitted on
   inactive tabs

dotnet build green (0 warnings); dotnet test 1,246 / 1,246 green
(243 Core.Net + 330 UI.Abstractions + 673 Core).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.UI.Abstractions/Panels/Settings/DisplaySettings.cs

@@ -0,0 +1,44 @@
+using System.Collections.Generic;
+
+namespace AcDream.UI.Abstractions.Panels.Settings;
+
+/// <summary>
+/// Display-related preferences persisted to <c>settings.json</c>.
+/// Modern addition (no retail equivalent for FOV / vsync etc) — replaces
+/// the various <c>ACDREAM_*</c> environment variables for resolution +
+/// windowed mode with an in-game UI.
+///
+/// <para>
+/// Records are immutable; mutation goes through
+/// <see cref="SettingsVM.SetDisplay"/> which assigns a new instance via
+/// <c>with</c>-expressions.
+/// </para>
+/// </summary>
+public sealed record DisplaySettings(
+    string Resolution,
+    bool Fullscreen,
+    bool VSync,
+    float FieldOfView,
+    float Gamma,
+    bool ShowFps)
+{
+    /// <summary>Values used on first launch / when settings.json is absent.</summary>
+    public static DisplaySettings Default { get; } = new(
+        Resolution:   "1920x1080",
+        Fullscreen:   false,
+        VSync:        true,
+        FieldOfView:  75f,
+        Gamma:        1.0f,
+        ShowFps:      false);
+
+    /// <summary>16:9 resolution presets offered in the dropdown.</summary>
+    public static IReadOnlyList<string> AvailableResolutions { get; } = new[]
+    {
+        "1280x720",
+        "1366x768",
+        "1600x900",
+        "1920x1080",
+        "2560x1440",
+        "3840x2160",
+    };
+}