[feature] Dedicated Watchlist view

## Summary
A view that shows only the stocks you've starred.

## Proposed change
A "Watchlist" section or route using `useWatchlist()`, reusing the screener row. Empty state when nothing is starred.

## Affected area
`app/screener` or a new route, `hooks/use-watchlist`.

## Acceptance criteria
- [ ] Only starred stocks appear; survives a reload
- [ ] Empty state guides you to star something

Start in `app/screener` and `hooks/use-watchlist`, then trace how `useWatchlist()` identifies starred stocks and how the screener row is rendered. Add a Watchlist section or route that reuses the row, handles an empty watchlist, and preserves starred stocks after reload. Verify both populated and empty states in the running app.

Written by the indexing model from the issue text.