Add startup visual modes and attempt registry

Implement startup visual behavior, de-duplicate startup attempts, and improve failure UX.

Key changes:
- Add spec and docs for startup visuals and timing contract (.trae/specs and docs/LAUNCHER_STARTUP_VISUALS.md).
- Introduce StartupVisualPreferences contract and resolver; create SplashWindow via resolved mode.
- Add StartupAttemptRecord model and a file-backed StartupAttemptRegistry to persist and coordinate in-progress startup attempts (attach/adopt, soft/hard timeouts, IPC/connect state, lifecycle updates).
- Update LauncherFlowCoordinator to: adopt/attach to existing attempts, track IPC connection and soft/hard timeouts (30s/120s), show delayed UI state, attempt foreground recovery via public IPC, compose detailed launch result metadata, and mark registry states (soft timeout, detached waiting, succeeded, failed).
- Add TryActivateExistingInstanceAsync to attempt activating an existing desktop via IPC.
- Change failure flow: ShowFailureWindowAsync now returns user choice; ErrorWindow updated to present Activate/Wait/Open Logs/Exit semantics and new layouts/styles; improved button wiring and debug/dev mode handling.
- Add UI and resource tweaks (ErrorWindow and SplashWindow changes), project asset link for nightly logo, and unit tests for StartupVisualPreferences.

These changes prevent duplicate desktop processes during slow startups, provide clearer UX for delayed startups, and persist startup attempt state across Launcher invocations for safer recovery/attach behavior.

LanMountainDesktop.Launcher/Models/StartupAttemptRecord.cs

@@ -0,0 +1,46 @@
+using System.Text.Json.Serialization;
+using LanMountainDesktop.Shared.Contracts.Launcher;
+
+namespace LanMountainDesktop.Launcher.Models;
+
+internal enum StartupAttemptState
+{
+    Pending,
+    SoftTimeout,
+    DetachedWaiting,
+    Succeeded,
+    Failed
+}
+
+internal sealed class StartupAttemptRecord
+{
+    [JsonPropertyName("attemptId")]
+    public string AttemptId { get; set; } = Guid.NewGuid().ToString("N");
+
+    [JsonPropertyName("hostPid")]
+    public int HostPid { get; set; }
+
+    [JsonPropertyName("startedAtUtc")]
+    public DateTimeOffset StartedAtUtc { get; set; } = DateTimeOffset.UtcNow;
+
+    [JsonPropertyName("updatedAtUtc")]
+    public DateTimeOffset UpdatedAtUtc { get; set; } = DateTimeOffset.UtcNow;
+
+    [JsonPropertyName("launchSource")]
+    public string LaunchSource { get; set; } = string.Empty;
+
+    [JsonPropertyName("successPolicy")]
+    public string SuccessPolicy { get; set; } = string.Empty;
+
+    [JsonPropertyName("lastObservedStage")]
+    public StartupStage LastObservedStage { get; set; } = StartupStage.Initializing;
+
+    [JsonPropertyName("lastObservedMessage")]
+    public string LastObservedMessage { get; set; } = string.Empty;
+
+    [JsonPropertyName("ipcConnected")]
+    public bool IpcConnected { get; set; }
+
+    [JsonPropertyName("state")]
+    public StartupAttemptState State { get; set; } = StartupAttemptState.Pending;
+}