Open the window on macOS by running gioapp.Main on the main thread

[Gustavobelfort]

Problem

On macOS, pitwall never opens a window. Fixes #122.

AppKit has to run [NSApp run] on the main thread, and Gio starts it from gioapp.Main(). cmd/pitwall never calls it, so newWindow waits forever for the app to finish launching.

Result

The GUI commands (pitwall, -s, --host) now go through onMain. On darwin it runs the command in a goroutine and calls gioapp.Main() on the main goroutine, the same way cmd/pwdemo-* do. It exits the process when the window closes. Other platforms behave as before. The existing error handling moved unchanged into exit().

Verification

OS and version	Architecture	Go version	Build command and result	Test commands and results
macOS 26.5.1	arm64	go1.27.1	GOFLAGS=-tags=novulkan go build ./cmd/pitwall: ok; go vet ./...: ok; gofmt -l: clean	go test ./internal/agent ./internal/pane ./internal/model ./internal/layout ./internal/input: all ok. Manual: the patched binary opens the window (display: AppKit, window focused: true in gui.log); the unpatched build does not.

go test ./cmd/pitwall (not in CONTRIBUTING's macOS list) fails the same 8 tests on unpatched main and on this branch (TestDialOrStart*, TestAgentTalk, TestRemoteReconnects, TestRunStaysOffHome, TestHookRefusalLostToReset), so this change doesn't cause them.

Platforms not tested: Linux, Windows. The non-darwin path calls the same function synchronously, as before.

• I listed the OS, architecture, and build/test results for each environment I tested.
• The change addresses one problem.
• Behavior changes have a regression test. (Needs an AppKit main thread; covered manually.)
• Documentation and third-party notices match the change.
• Logs, screenshots and fixtures contain no private data or credentials.

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