docs: add JSDoc categories to @fresh/core public API

[SisyphusZheng]

What

Adds @module JSDoc block and @category tags to the @fresh/core public API entry point (packages/fresh/src/mod.ts).

All 30+ public exports are grouped into three categories:

Category	Exports
Core	App, ListenOptions, Context, FreshContext, Island, WebSocketHandlers, WebSocketUpgradeOptions, FreshConfig, ResolvedFreshConfig, HttpError, PageProps
Routing	page, HandlerFn, HandlerByMethod, PageResponse, RouteData, RouteHandler, LayoutConfig, Lazy, MaybeLazy, RouteConfig, Method, createDefine, Define
Middleware	Middleware, MiddlewareFn, railingSlashes, staticFiles, csrf/CsrfOptions, cors/CORSOptions, ipFilter/IpFilterOptions/IpFilterRules, csp/CSPOptions

Why

JSR doc score for @fresh/core reports poorly documented public symbols. This change provides structured navigation and improves discoverability without any runtime changes.

Refs #3596

Verification

Docs-only change. deno task check:types passes. No runtime behavior affected.