feat: replace setup with init wizard + interactive config menu

- `unarr init` (alias: `unarr setup`): streamlined 3-step wizard
  (API key, download dir, daemon install). Removed method/name prompts
  — auto-configured from defaults.
- `unarr config [category]`: interactive menu with 7 categories
  (downloads, organization, notifications, device, region, connection,
  advanced). Direct access via `unarr config downloads`, etc.
- Extract shared helpers (openBrowser, expandHome, isTerminal) to
  helpers.go. Delete old setup.go and config.go.
- Update all "unarr setup" references to "unarr init" across daemon,
  doctor, status, README, install scripts.

internal/cmd/status.go

@@ -33,7 +33,7 @@ func runStatus() error {
 	cfg := loadConfig()
 
 	if cfg.Auth.APIKey == "" {
-		dim.Println("  Not configured. Run 'unarr setup' first.")
+		dim.Println("  Not configured. Run 'unarr init' first.")
 		fmt.Println()
 		return nil
 	}