Add a JWT Decoder component to the app

[devvsakib]

Description:
Add a small, secure, client-side JWT Decoder component that lets developers paste a JWT and see the decoded header and payload (pretty-printed), plus the raw signature and basic validation checks. This is intended as a dev utility (not for sensitive tokens). The component should NOT attempt to verify signatures (no secret keys on client) — only decode base64url sections and show parsing errors clearly.

Acceptance criteria:

• A React component named JWTDecoder is added and exported as default.
• Allows pasting a JWT (single-line string).
• Decodes header and payload (base64url) and shows them as pretty-printed JSON.
• Shows the signature separately (raw base64url) and indicates whether the token has 3 parts.
• Handles and shows decoding/parsing errors clearly.
• Provides buttons for "Decode", "Clear", and "Copy Payload as JSON".
• Uses no back-end. No signature verification is performed.
• Includes unit-testable pure utility functions for decoding base64url and parseJWT.

Steps to test:

1. Run the dev server and open the dev tools page containing the JWT Decoder.
2. Paste a valid JWT (e.g. eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...) and click Decode.
3. Confirm header and payload show as JSON, signature shown, and no errors.
4. Paste invalid strings and confirm errors display.