features/watchlist/lib/fetchers.ts: reuse existing fetcher; drop unnecessary type casts

[electather]

Part of epic #491. Review comments:

• apps/client/src/features/watchlist/lib/fetchers.ts:21 — "this function already exists. use it"
• apps/client/src/features/watchlist/lib/fetchers.ts:75 — "is the type cast necessary for these functions?"

Acceptance criteria

• Fetcher at :21 replaced with existing implementation (likely from api.* Hono client or shared transport).
• Type casts at :75 removed if redundant; if needed, comment explains why.
• vp check + vp test pass.