Add RESERVED_SLUGS constant and consolidate exports

Introduce a RESERVED_SLUGS constant to standardize restricted slugs across the application. Created a `constants/index.ts` file to centralize and streamline exports for better maintainability.

frontend/src/constants/common.ts

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+export const RESERVED_SLUGS = ["new", "edit", "delete", "settings"];

frontend/src/constants/index.ts

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+export * from "./common";