docs: improve CLI help, shell completion, and README

- Add command groups (Getting Started, Search, Downloads, Daemon, System)
- Add shell completion command (bash, zsh, fish, powershell)
- Add flag completions for --type, --quality, --sort, --lang, --genre,
  --country, --method, --player
- Improve Long descriptions and Examples for all commands
- Split doctor disk check into platform-specific files (Unix/Windows)
- Validate infoHash length before truncating (prevent panic)
- Fix references to non-existent 'unarr daemon start' command
- Move stats command to System & Diagnostics group
- Rewrite README with complete documentation, correct config format
  (toml not yaml), all commands, shell completion section

internal/cmd/doctor_windows.go

@@ -0,0 +1,33 @@
+//go:build windows
+
+package cmd
+
+import (
+	"fmt"
+	"syscall"
+	"unsafe"
+)
+
+func checkDiskSpace(dir string) (string, error) {
+	kernel32 := syscall.NewLazyDLL("kernel32.dll")
+	getDiskFreeSpaceEx := kernel32.NewProc("GetDiskFreeSpaceExW")
+
+	var freeBytesAvailable, totalBytes, totalFreeBytes int64
+	dirPtr, _ := syscall.UTF16PtrFromString(dir)
+	ret, _, err := getDiskFreeSpaceEx.Call(
+		uintptr(unsafe.Pointer(dirPtr)),
+		uintptr(unsafe.Pointer(&freeBytesAvailable)),
+		uintptr(unsafe.Pointer(&totalBytes)),
+		uintptr(unsafe.Pointer(&totalFreeBytes)),
+	)
+	if ret == 0 {
+		return "", fmt.Errorf("GetDiskFreeSpaceEx: %w", err)
+	}
+
+	gb := float64(freeBytesAvailable) / (1024 * 1024 * 1024)
+	msg := fmt.Sprintf("%.1f GB free", gb)
+	if gb < 10 {
+		return "!" + msg + " (low)", nil
+	}
+	return msg, nil
+}