install.ps1: use 'return' over 'exit 1' for Install-UnslothStudio bail-outs

Per Gemini review #4177659001: when users run install.ps1 via
'irm ... | iex', 'exit 1' inside the function terminates the entire
PowerShell process and closes the user's terminal. 'return' bails out
of the function while keeping the shell open, matching existing error
sites at lines 34, 50, 57.

Three sites fixed: --tauri+env-override guard, env-override mkdir/access
failure, and write-probe failure. The 'exit' calls at lines 591/611
are inside a generated launcher here-string (a separate top-level .ps1
that runs as its own process), so they correctly stay as 'exit'.

install.ps1

@@ -96,7 +96,7 @@ function Install-UnslothStudio {
             Write-Host "       The desktop app still uses the legacy %USERPROFILE%\.unsloth\studio root." -ForegroundColor Red
             Write-Host "       Run install.ps1 without --tauri for custom-root shell installs," -ForegroundColor Yellow
             Write-Host "       or unset the env var for default desktop installs." -ForegroundColor Yellow
-            exit 1
+            return
         }
     }
 
@@ -121,7 +121,7 @@ function Install-UnslothStudio {
             $StudioHome = (Resolve-Path -LiteralPath $envOverride).Path
         } catch {
             Write-Host "ERROR: STUDIO_HOME=$envOverride cannot be created or accessed." -ForegroundColor Red
-            exit 1
+            return
         }
         $probe = Join-Path $StudioHome (".unsloth-write-probe-" + [guid]::NewGuid())
         try {
@@ -130,7 +130,7 @@ function Install-UnslothStudio {
             Remove-Item -LiteralPath $probe -Force -ErrorAction SilentlyContinue
         } catch {
             Write-Host "ERROR: STUDIO_HOME=$StudioHome is not writable." -ForegroundColor Red
-            exit 1
+            return
         }
         $StudioDataDir = Join-Path $StudioHome "share"
         $StudioRedirectMode = 'env'