feat(frontend): add watchlist for tracked items and worlds

[Flickwire-Agent]

User Story

As a regular user, I want a watchlist of items and worlds I care about so that I can focus on markets I actually trade instead of scanning the full opportunity list every time.

Why This Matters

Players often specialise in a few item categories, home worlds, or retainers. A watchlist gives the app a personalised workflow without requiring accounts.

Acceptance Criteria

• Users can add an item to a watchlist from opportunity rows, bargain rows, item detail pages, and listing pages.
• Users can remove watched items from the same places and from a dedicated watchlist view.
• Users can optionally store preferred buy worlds/data centers and sell worlds/data centers.
• The opportunities page can filter to watched items only.
• Watched state persists across browser sessions.
• Empty-watchlist and first-use states explain how to add items.
• The implementation does not require server-side authentication.

Useful Context

• Item detail, listings, bargains, DC disparities, and opportunities routes already exist in the SPA.
• A localStorage-backed model is acceptable for the first iteration.
• This should compose with saved filtered views and future alert rules.

Suggested Verification

• Build the web app.
• Manually test watch/unwatch from each relevant page.
• Refresh the browser and confirm persistence.