[Feature] Add encoder and decoder utility

[yeleepark]

Summary

Add a utility for encoding and decoding common text formats such as URL encoding, Base64, and HTML entities.

Why

Encoding and decoding small text snippets is a common workflow for developers, marketers, and anyone working with URLs, query strings, embeds, or API payloads. It is lightweight, useful, and fits oddkit's local browser tool model.

Proposed scope

• URL encode and decode text
• Base64 encode and decode text
• HTML entity encode and decode text
• Copy output to clipboard
• Show clear errors for invalid decode input
• Process everything locally in the browser

UX notes

• Use a mode selector for the encoding type
• Keep input and output visible at the same time
• Suggested microcopy: // make text survive the wire

Acceptance criteria

• URL, Base64, and HTML entity modes work in both directions
• Invalid decode input shows a useful error state
• Output can be copied to the clipboard
• No input text is uploaded to a server