fix(webapp): declare @growthbook type shim for bundler resolution

Under `moduleResolution: "bundler"` (introduced for webapp in #5952), TS
respects each package's `exports` map. The growthbook packages only expose
`require`/`import` conditions, so the .d.ts files in `dist/` are no longer
reachable and the strict typecheck flags every file that imports types
from them. Declare the small subset (JSONValue, WidenPrimitives,
FeatureDefinition, GrowthBook, hooks) that the codebase actually consumes
in the webapp's custom.d.ts.

Author: davidercruz

packages/webapp/custom.d.ts

@@ -4,3 +4,53 @@ declare module '*.svg' {
   const value: SvgrComponent;
   export default value;
 }
+
+// `@growthbook/growthbook(-react)` ship .d.ts files in their `dist/` folders,
+// but their package.json `exports` map only lists "require"/"import" — under
+// `moduleResolution: "bundler"` TypeScript respects `exports` and can no
+// longer reach the typings. Declare the subset we actually consume so type
+// imports keep resolving without falling back to `any`.
+declare module '@growthbook/growthbook' {
+  export type JSONValue =
+    | null
+    | number
+    | string
+    | boolean
+    | Array<JSONValue>
+    | { [key: string]: JSONValue };
+  export type WidenPrimitives<T> = T extends string
+    ? string
+    : T extends number
+    ? number
+    : T extends boolean
+    ? boolean
+    : T;
+  export interface FeatureDefinition<T = unknown> {
+    defaultValue?: T;
+    rules?: unknown[];
+  }
+  export type Experiment = Record<string, unknown>;
+  export type Result = Record<string, unknown>;
+  export interface GrowthBook {
+    [key: string]: unknown;
+  }
+  export const GrowthBook: {
+    new (options?: unknown): GrowthBook;
+  };
+}
+
+declare module '@growthbook/growthbook-react' {
+  export interface GrowthBook {
+    [key: string]: unknown;
+  }
+  export const GrowthBook: {
+    new (options?: unknown): GrowthBook;
+  };
+  export const GrowthBookProvider: React.FC<{
+    children?: React.ReactNode;
+    growthbook?: GrowthBook;
+  }>;
+  export function useFeatureValue<T>(key: string, fallback: T): T;
+  export function useGrowthBook(): GrowthBook | undefined;
+  export function useFeatureIsOn(key: string): boolean;
+}