feat: initialState to be derived from Context

Useful with path parameters.

src/frog-base.tsx

@@ -146,7 +146,7 @@ export type FrogConstructorParameters<
      * }
      * ```
      */
-    initialState?: _state | undefined
+    initialState?: ((c: Context<env>) => _state) | _state | undefined
     /**
      * Origin URL of the server instance.
      *
@@ -264,7 +264,7 @@ export class FrogBase<
 > {
   // Note: not using native `private` fields to avoid tslib being injected
   // into bundled code.
-  _initialState: env['State'] = undefined as env['State']
+  _initialState: ((c: Context<env>) => _state) | _state | undefined = undefined
   /** Path for assets. */
   assetsPath: string
   /** Base path of the server instance. */
@@ -585,7 +585,10 @@ export class FrogBase<
           secret: this.secret,
           verify,
         }),
-        initialState: this._initialState,
+        initialState:
+          typeof this._initialState === 'function'
+            ? (this._initialState as any)(c)
+            : this._initialState,
         origin,
       })
 
@@ -989,7 +992,10 @@ export class FrogBase<
         context: await requestBodyToImageContext(c, {
           secret: this.secret,
         }),
-        initialState: this._initialState,
+        initialState:
+          typeof this._initialState === 'function'
+            ? (this._initialState as any)(c)
+            : this._initialState,
       })
 
       const response = await handler(context)

src/frog.tsx

@@ -40,7 +40,7 @@ export class Frog<
   //
   _state = env['State'],
 > extends FrogBase<env, schema, basePath, _state> {
-  constructor(params: FrogConstructorParameters) {
+  constructor(params: FrogConstructorParameters<env, basePath, _state>) {
     super(params as any)
 
     const frame = this.frame