Add 'furlpay invest watchlist' command

[Ashutosh0x]

The CLI covers buy/sell/portfolio/quote/dca but has no watchlist management.

Task
Add furlpay invest watchlist [add <SYMBOL> | remove <SYMBOL>] in lib/commands.js (see the invest dispatcher). Store the watchlist locally in ~/.furlpay/config.json (the API has no watchlist endpoint yet), and render it with quotes fetched from GET /api/markets/<symbol> using the existing render() table helper in lib/output.js.

Acceptance

• watchlist add TSLA / remove TSLA mutate config and print a confirmation.
• watchlist with no args lists symbols with live price and daily change.
• Honors --output json|csv and --quiet like every other listing command.
• Zero new dependencies.