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/setup.go

@@ -23,7 +23,17 @@ func newSetupCmd() *cobra.Command {
 	cmd := &cobra.Command{
 		Use:   "setup",
 		Short: "First-time configuration wizard",
-		Long:  "Interactive setup that configures API key, download directory, and preferred download method.",
+		Long: `Interactive setup that configures API key, download directory, and
+preferred download method.
+
+Opens your browser to create/copy your API key, then walks you through
+choosing a download directory, method (torrent, debrid, usenet), and
+device name. Validates the API key against the server before saving.
+
+Run this once after installing unarr. To change settings later,
+use 'unarr config' or edit ~/.config/unarr/config.toml directly.`,
+		Example: `  unarr setup
+  unarr setup --api-url https://custom.server.com`,
 		RunE: func(cmd *cobra.Command, args []string) error {
 			return runSetup(apiURL)
 		},
@@ -238,7 +248,7 @@ func runSetup(apiURLOverride string) error {
 	}
 	cyan.Printf("  Available: %s\n", strings.Join(features, ", "))
 	fmt.Println()
-	fmt.Println("  Next: run", bold.Sprint("unarr daemon start"), "to begin downloading")
+	fmt.Println("  Next: run", bold.Sprint("unarr start"), "to begin downloading")
 	fmt.Println()
 
 	return nil