Config Checkpoints dialog + daily auto-checkpoint; drop installer GUI launch

Three fixes:

1. Config Checkpoints submenu replaced with a proper dialog. Lists
   checkpoints with timestamp/size/filename, has a "Take Checkpoint
   Now" button, and a "Roll Back" button that becomes enabled when a
   row is selected. The previous click-a-menu-entry-immediate-restore
   flow was too easy to fire by accident.

2. New CheckpointScheduler BackgroundService creates a checkpoint at
   midnight every day. Combined with the existing auto-on-save
   snapshots, this guarantees a daily rollback point even if the
   config wasn't edited that day. A new "create-checkpoint" admin op
   plus AdminPipeServer.CreateCheckpoint helper does the actual file
   copy; both manual (via the dialog) and the scheduler use it.

3. Installer: drop the post-install "Launch Webhook Server" wizard
   step. It tried to launch the GUI un-elevated, which fails because
   the GUI's manifest is requireAdministrator. The Start Menu shortcut
   handles elevation correctly, so the user can launch from there.

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

installer/webhook-server.iss

@@ -69,9 +69,10 @@ Filename: "powershell.exe"; \
     Parameters: "-NoProfile -ExecutionPolicy Bypass -File ""{app}\scripts\install-service.ps1"" -BinaryPath ""{app}\{#ServiceExeName}"""; \
     StatusMsg: "Installing Windows Service..."; \
     Flags: runhidden
-Filename: "{app}\{#AppExeName}"; \
-    Description: "Launch {#AppName}"; \
-    Flags: postinstall nowait skipifsilent
+; No post-install GUI launch: the GUI is requireAdministrator and the launch
+; from the installer wizard ends up un-elevated for the post-install user, so
+; it would just fail to connect to the admin pipe. The Start Menu shortcut
+; handles the elevation correctly via the embedded manifest.
 
 [UninstallRun]
 Filename: "powershell.exe"; \