CI: install MSVC build tools on Windows runners that lack them

build-windows failed on this PR with "linker `link.exe` not found",
while build-linux and build-container passed — the code was fine, the
runner environment was not.

The job installs Rust and Node conditionally but assumed the MSVC C++
toolchain was hand-provisioned. A runner without it registers normally,
advertises windows-latest, accepts the job, downloads the entire crate
graph and only then fails at link time. That also means a bare runner
coming online turns a job that would have queued for a capable machine
into a failed build.

Installs the VC++ workload when vswhere cannot find it, matching the
existing conditional Rust and Node steps. rustc locates MSVC through
vswhere and the registry rather than PATH, so no dev-shell activation is
needed. Installer exit 3010 (success, reboot pending) is treated as
success.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.gitea/workflows/build-app.yml

@@ -357,6 +357,38 @@ jobs:
           (Get-Content app/src-tauri/Cargo.toml) -replace '^version = ".*?"', "version = `"$version`"" | Set-Content app/src-tauri/Cargo.toml
           Write-Host "Patched version to $version"
 
+      - name: Install MSVC C++ build tools
+        shell: cmd
+        run: |
+          rem Tauri links with MSVC, so rustc needs link.exe and the Windows SDK.
+          rem This job previously assumed a hand-provisioned runner; a runner
+          rem without them registers fine, accepts windows-latest jobs, and then
+          rem fails at link time with "linker `link.exe` not found" after having
+          rem already downloaded the whole crate graph.
+          rem
+          rem rustc locates MSVC through vswhere/the registry rather than PATH,
+          rem so installing is sufficient — no dev-shell activation needed here.
+          set "VSWHERE=%ProgramFiles(x86)%\Microsoft Visual Studio\Installer\vswhere.exe"
+          if exist "%VSWHERE%" (
+            "%VSWHERE%" -latest -products * -requires Microsoft.VisualStudio.Component.VC.Tools.x86.x64 -property installationPath > "%TEMP%\vcpath.txt" 2>nul
+            for /f "usebackq delims=" %%i in ("%TEMP%\vcpath.txt") do set "VCPATH=%%i"
+          )
+          if defined VCPATH (
+            echo MSVC build tools already present at %VCPATH%
+          ) else (
+            echo MSVC build tools not found - installing Visual Studio Build Tools
+            curl -fSL -o "%TEMP%\vs_BuildTools.exe" https://aka.ms/vs/17/release/vs_BuildTools.exe || exit /b 1
+            rem 3010 means "installed, reboot pending" and is a success for our purposes.
+            "%TEMP%\vs_BuildTools.exe" --quiet --wait --norestart --nocache ^
+              --add Microsoft.VisualStudio.Workload.VCTools --includeRecommended
+            set "VSEXIT=%ERRORLEVEL%"
+            del "%TEMP%\vs_BuildTools.exe" 2>nul
+            if not "%VSEXIT%"=="0" if not "%VSEXIT%"=="3010" (
+              echo Visual Studio Build Tools installer failed with %VSEXIT%
+              exit /b %VSEXIT%
+            )
+            echo Visual Studio Build Tools installed
+          )
       - name: Install Rust stable
         run: |
           where rustup >nul 2>&1 && (