Self-update: UAC-aware, de-elevated relaunch, user-mode installer

Three coordinated changes so the auto-update flow works on every
existing install location.

LauncherSelfUpdater.cs
----------------------
- Probe the target directory with a write/delete of a temp file. If
  it fails (e.g. install in Program Files without admin), set
  UseShellExecute=true + Verb=runas on the Updater process so UAC
  prompts the user once for elevation. If the directory is writable
  (user-mode install or portable layout), skip elevation entirely.
- Switched from ProcessStartInfo.ArgumentList to a quoted Arguments
  string because Verb=runas requires UseShellExecute=true, which
  ignores ArgumentList. Paths get explicit quotes to survive spaces.

Updater Program.cs
------------------
After the file swap, the relaunch was a direct Process.Start(target).
With UAC elevation that propagates admin to the new launcher, then to
its child PROSERVE_UE_5_5.exe — undesirable. Replace with
`explorer.exe "<target>"`: explorer is always running in the user's
normal token, opens the exe via shell, the new process inherits the
non-elevated session. Standard de-elevation trick.

installer/PSLauncher.iss
------------------------
Switch from system-wide install (Program Files, requires admin) to
per-user (DefaultDirName={localappdata}\Programs\..., PrivilegesRequired=
lowest). Auto-update then never needs UAC at all on fresh installs.
Existing installs in Program Files keep working thanks to the runas
fallback above.

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

installer/PSLauncher.iss

@@ -28,7 +28,9 @@ AppPublisher={#MyAppPublisher}
 AppPublisherURL={#MyAppURL}
 AppSupportURL={#MyAppURL}
 AppUpdatesURL={#MyAppURL}
-DefaultDirName={autopf}\{#MyAppPublisher}\{#MyAppShortName}
+; Installation user-mode (pas d'admin requis) : le launcher pourra se mettre à jour
+; tout seul via PSLauncher.Updater.exe sans déclencher de UAC.
+DefaultDirName={localappdata}\Programs\{#MyAppPublisher}\{#MyAppShortName}
 DefaultGroupName={#MyAppName}
 DisableProgramGroupPage=yes
 OutputDir=output
@@ -37,8 +39,9 @@ SetupIconFile=
 Compression=lzma2/ultra
 SolidCompression=yes
 WizardStyle=modern
-PrivilegesRequired=admin
+PrivilegesRequired=lowest
 PrivilegesRequiredOverridesAllowed=dialog
+UsedUserAreasWarning=no
 ArchitecturesAllowed=x64
 ArchitecturesInstallIn64BitMode=x64
 UninstallDisplayName={#MyAppName}