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/completion_helpers.go

@@ -0,0 +1,14 @@
+package cmd
+
+import "github.com/spf13/cobra"
+
+// completionCountryCodes provides shell completion for --country flags.
+func completionCountryCodes(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
+	return []string{
+		"US\tUnited States", "GB\tUnited Kingdom", "ES\tSpain", "FR\tFrance",
+		"DE\tGermany", "IT\tItaly", "PT\tPortugal", "BR\tBrazil",
+		"MX\tMexico", "AR\tArgentina", "CA\tCanada", "AU\tAustralia",
+		"NL\tNetherlands", "SE\tSweden", "NO\tNorway", "DK\tDenmark",
+		"FI\tFinland", "JP\tJapan", "KR\tSouth Korea", "IN\tIndia",
+	}, cobra.ShellCompDirectiveNoFileComp
+}