fix(ui): bound the ConPTY launch-gate wait so a hung launch can't freeze other panes

StartAsync awaited the static launch-gate semaphore with no timeout, so one
hung launch (slow disk, AV scanning claude.exe, a Porta.Pty/ConPTY hiccup)
blocked every subsequent pane open behind it indefinitely, including Retry.
Extracted the wait into WaitForLaunchGateAsync with a 30s timeout that
throws TimeoutException before the try/finally (never releasing a gate it
didn't acquire); the exception flows through StartCoreAsync's existing
catch into StartError, so the pane shows an error + Retry instead of
hanging. StartError renders ex.Message directly (no locale key involved),
so no locale changes were needed.

PtyTerminalSession.StartAsync itself needs a real TerminalControl and stays
impractical to unit-test directly, so tests target the extracted
WaitForLaunchGateAsync helper against a plain SemaphoreSlim: timeout throws,
timeout never acquires the gate, and success still acquires it.

src/ClaudeDo.Ui/Services/PtyTerminalSession.cs

@@ -25,7 +25,21 @@ public sealed class PtyTerminalSession : IDisposable
     // inherits the other's env (e.g. CLAUDEDO_PLANNING_TOKEN, breaking that session's own MCP
     // auth). Process-wide env leakage AFTER a launch has forked remains a documented limitation
     // — Porta.Pty has no per-launch env seam, so the vars stay set on the whole UI process.
+    // The wait is bounded (see WaitForLaunchGateAsync) — a hung launch (slow disk, AV scanning
+    // claude.exe, a Porta.Pty/ConPTY hiccup) must not freeze every other pane open behind it.
     private static readonly SemaphoreSlim s_launchGate = new(1, 1);
+    private static readonly TimeSpan s_launchGateTimeout = TimeSpan.FromSeconds(30);
+
+    /// <summary>
+    /// Waits on <paramref name="gate"/> for at most <paramref name="timeout"/>, throwing
+    /// <see cref="TimeoutException"/> instead of blocking forever. Never acquires the gate on
+    /// timeout, so callers must not release it in that case.
+    /// </summary>
+    internal static async Task WaitForLaunchGateAsync(SemaphoreSlim gate, TimeSpan timeout, CancellationToken ct)
+    {
+        if (!await gate.WaitAsync(timeout, ct))
+            throw new TimeoutException("Another terminal launch is still starting up. Please retry in a moment.");
+    }
 
     private TerminalControl? _control;
     private bool _disposed;
@@ -50,7 +64,7 @@ public sealed class PtyTerminalSession : IDisposable
         control.Args = new List<string>(descriptor.Args);
         control.StartingDirectory = descriptor.Cwd;
 
-        await s_launchGate.WaitAsync(ct);
+        await WaitForLaunchGateAsync(s_launchGate, s_launchGateTimeout, ct);
         try
         {
             foreach (var (key, value) in descriptor.Env)