feat: add --quiet / -q flag to suppress TUI output

[Jefsky]

Summary

Adds a --quiet / -q flag to baa that skips the TUI and runs package manager updates directly in the background. Only errors are printed; successful updates produce no output.

Changes

• Added quiet flag (--quiet, -q) in main.go
• Updated flag.Usage to document the new flag
• Added runQuietMode() function that:
  • Detects installed package managers
  • Runs updates for each manager in background goroutines
  • Silently drains output (no TUI)
  • Only prints errors to stderr
• If no package managers are found, exits silently (not an error)

Behavior

# Normal mode (TUI)
baa

# Quiet mode (background, errors only)
baa --quiet
# or
baa -q

In quiet mode:

• No TUI is displayed
• Package manager output is suppressed
• Only errors are printed to stderr
• Exit code 0 on success, 1 on error

Closes #2