scaffold Flutter Windows desktop target

Add the generated Windows runner tree and register the windows
platform in .metadata. The Flutter SDK revision bump that came with
`flutter create` for windows also re-resolved the transitive pins in
pubspec.lock. .gitignore now excludes the built clide-cli binary on
Windows, matching the existing linux/macos entries.

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

.gitignore

@@ -58,6 +58,8 @@ tools/ui/.serve.pid
 /native/linux-x64/clide
 /native/macos-arm64/clide
 /native/macos-x64/clide
+/native/windows-x64/clide.exe
+/native/windows-x64/clide.obj
 
 # -- Test, coverage, profile output ------------------------------------
 *.test