Add light/dark theme toggle with system preference support

[Copilot]

Adds user-controllable theme switching that cycles between System (follows OS), Light, and Dark modes. Preference persists via cookies.

Implementation

• ThemeService: Manages theme state with three modes, cookie persistence, and event-driven updates
• MainLayout.razor: Icon button in app bar cycles through modes with contextual tooltips
• WellKnownCookieValues: Extended with theme preference helpers
• DI Registration: ThemeService registered as scoped service

Usage

// ThemeService exposes current state and handles transitions
public interface IThemeService
{
    ThemePreference CurrentPreference { get; }  // System | Light | Dark
    bool IsDarkMode { get; }  // Computed from preference + OS setting
    event EventHandler? ThemeChanged;
    Task CycleThemeAsync();  // System → Light → Dark → System
}

Icon reflects current mode: Brightness4 (System) → LightMode → DarkMode. Tooltip shows current mode and next action.

Testing

14 unit tests cover initialization, mode transitions, cookie persistence, event propagation, and system preference updates.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add light/dark theme toggle</issue_title>
<issue_description>MudBlazor supports toggling between light and dark theme.
The default behavior should be to follow the system setting, but allow the user to manually toggle between light and dark.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add light/dark theme toggle #457

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[github-actions]

Summary - Code Coverage

Summary

Generated on:	11/18/2025 - 06:32:47
Coverage date:	11/18/2025 - 06:32:38 - 11/18/2025 - 06:32:43
Parser:	MultiReport (3x Cobertura)
Assemblies:	3
Classes:	26
Files:	24
Line coverage:	60.1% (424 of 705)
Covered lines:	424
Uncovered lines:	281
Coverable lines:	705
Total lines:	1683
Branch coverage:	48.5% (173 of 356)
Covered branches:	173
Total branches:	356
Method coverage:	Feature is only available for sponsors
Tag:	961_19456551077

Coverage

PointerStar.Client - 46.3%

Name	Line	Branch
PointerStar.Client	46.3%	41.4%
PointerStar.Client.Components.ComponentBase`1	0%
PointerStar.Client.Components.QRCode	0%	0%
PointerStar.Client.Components.UserDialog	0%	0%
PointerStar.Client.Cookies.Cookie	0%	0%
PointerStar.Client.Cookies.WellKnownCookieValues	100%	100%
PointerStar.Client.Pages.Index	0%
PointerStar.Client.Pages.Room	0%	0%
PointerStar.Client.Services.ThemeService	96.9%	90.9%
PointerStar.Client.Shared.MainLayout	0%	0%
PointerStar.Client.ViewModels.ClipboardService	0%
PointerStar.Client.ViewModels.RoomViewModel	98.1%	88.5%
PointerStar.Client.ViewModels.UserDialogViewModel	93.7%	100%
PointerStar.Client.ViewModels.ViewModelBase	100%
Program	0%

PointerStar.Server - 83.1%

Name	Line	Branch
PointerStar.Server	83.1%	66.6%
AspNetCoreGeneratedDocument.Pages_Error	0%	0%
PointerStar.Server.Controllers.RoomController	100%
PointerStar.Server.Hubs.RoomHub	0%	0%
PointerStar.Server.Pages.ErrorModel	0%	0%
PointerStar.Server.Room.InMemoryRoomManager	99.4%	98.5%
Program	94.1%	50%

PointerStar.Shared - 48.1%

Name	Line	Branch
PointerStar.Shared	48.1%	37.5%
PointerStar.Shared.Role	78.5%	50%
PointerStar.Shared.RoomHubConnection	0%	0%
PointerStar.Shared.RoomOptions	100%
PointerStar.Shared.RoomState	100%
PointerStar.Shared.User	100%
PointerStar.Shared.UserOptions	100%