Remove standalone fetcher, add setup tool with install/uninstall workflow

Drop claude-fetcher binary and cron job — the widget's built-in
BackgroundFetcher is the sole fetcher now. Add cmd/setup with cross-platform
install and uninstall (--uninstall): kills widget, removes binaries + autostart,
cleans Claude Code statusline setting, optionally removes config dir.

Also includes: browser-based login (chromedp), ICO wrapper for Windows tray
icon, and reduced icon size (64px).

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

cmd/setup/ui_windows.go

@@ -0,0 +1,23 @@
+//go:build windows
+
+package main
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+var (
+	user32     = syscall.NewLazyDLL("user32.dll")
+	procMsgBox = user32.NewProc("MessageBoxW")
+)
+
+func showMessage(title, text string, isError bool) {
+	var flags uintptr = 0x00000040 // MB_OK | MB_ICONINFORMATION
+	if isError {
+		flags = 0x00000010 // MB_OK | MB_ICONERROR
+	}
+	tPtr, _ := syscall.UTF16PtrFromString(title)
+	mPtr, _ := syscall.UTF16PtrFromString(text)
+	procMsgBox.Call(0, uintptr(unsafe.Pointer(mPtr)), uintptr(unsafe.Pointer(tPtr)), flags)
+}