feat: add URL Encoder / Decoder tool

[sg172003]

Problem Statement

Encoding and decoding URL strings is a frequent task when working with APIs, query parameters, and redirects. No client-side tool exists for this in DevUtils currently.

Proposed Solution

Add a URL Encoder / Decoder page with:

• Encode strings using encodeURIComponent
• Decode using decodeURIComponent
• Graceful error handling for malformed input
• Copy output to clipboard
• Utility functions in src/utils/urlUtils.ts
• Unit tests in src/tests/urlUtils.test.ts

Alternatives Considered

None — encodeURIComponent and decodeURIComponent are native browser APIs, no library needed.