fix: Windows download — TLS12 + WebClient fallback

Invoke-WebRequest fails without TLS12 on some Windows.
Use System.Net.WebClient with explicit TLS12 and User-Agent.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

full_install.ps1

@@ -74,6 +74,9 @@ New-Item -ItemType Directory -Path $localDir | Out-Null
 $scriptDir = Split-Path -Parent $MyInvocation.MyCommand.Path
 if (-not $scriptDir) { $scriptDir = Get-Location }
 
+# Ensure TLS 1.2 for GitHub downloads
+[Net.ServicePointManager]::SecurityProtocol = [Net.SecurityProtocolType]::Tls12
+
 $filesToGet = @("ssh_manager.py", "gen_icon.py")
 foreach ($f in $filesToGet) {
     $localFile = Join-Path $scriptDir $f
@@ -84,10 +87,20 @@ foreach ($f in $filesToGet) {
     } else {
         Write-Host "[*] Downloading $f from GitHub..." -ForegroundColor Yellow
         try {
-            Invoke-WebRequest -Uri "$REPO_RAW/$f" -OutFile $destFile -UseBasicParsing
-            Write-Host "[OK] $f (downloaded)" -ForegroundColor Green
+            $url = "$REPO_RAW/$f"
+            Write-Host "    URL: $url" -ForegroundColor DarkGray
+            $wc = New-Object System.Net.WebClient
+            $wc.Headers.Add("User-Agent", "PCA-SSH-Installer")
+            $wc.DownloadFile($url, $destFile)
+            if (Test-Path $destFile) {
+                $sz = (Get-Item $destFile).Length
+                Write-Host "[OK] $f ($sz bytes)" -ForegroundColor Green
+            } else {
+                throw "File not saved"
+            }
         } catch {
-            Write-Host "[FAIL] Cannot download $f" -ForegroundColor Red
+            Write-Host "[FAIL] Cannot download $f : $_" -ForegroundColor Red
+            Write-Host "Try manual download: https://github.com/andrey271192/ssh-manage" -ForegroundColor Yellow
             Read-Host "Press Enter"
             exit 1
         }

install_windows.txt

@@ -5,7 +5,7 @@
 СПОСОБ 1 — Одна команда (рекомендуется)
 Скопируй и вставь в PowerShell:
 
-irm https://raw.githubusercontent.com/andrey271192/ssh-manage/main/full_install.ps1 -OutFile $env:TEMP\pca_install.ps1; powershell -ExecutionPolicy Bypass -File $env:TEMP\pca_install.ps1
+[Net.ServicePointManager]::SecurityProtocol=[Net.SecurityProtocolType]::Tls12; $wc=New-Object Net.WebClient; $wc.Headers.Add('User-Agent','PCA'); $wc.DownloadFile('https://raw.githubusercontent.com/andrey271192/ssh-manage/main/full_install.ps1',"$env:TEMP\pca_install.ps1"); powershell -ExecutionPolicy Bypass -File $env:TEMP\pca_install.ps1
 
 Что произойдёт:
 - Скачает установщик с GitHub