fix(js): add typed ChallengeResult interface for PoW worker results

The process() function in fast.ts declared Promise<string> but actually
resolved with the full worker message object { hash, data, difficulty,
nonce }. main.tsx papered over this with `any`. Introduce a shared
ChallengeResult type so the contract between workers, algorithms, and
the main component is enforced by the type checker.

Assisted-by: Claude Opus 4.6 via Claude Code
Signed-off-by: Xe Iaso <me@xeiaso.net>

web/js/algorithms/index.ts

@@ -1,5 +1,7 @@
 import fast from "./fast";
 
+export type { ChallengeResult } from "./types";
+
 export default {
   fast: fast,
   slow: fast, // XXX(Xe): slow is deprecated, but keep this around in case anything goes bad