[Feature Request] Add AtCoder submission support

[Anjalihere]

Description

Currently, cph-submit works wonderfully for submitting Codeforces solutions directly from VS Code. It would be an amazing quality-of-life improvement if we could get the same auto-submit functionality for AtCoder.

AtCoder is one of the most popular platforms for competitive programming, and manually copying and pasting code from the editor to the browser during contests can break the workflow.

Proposed Solution

• Extend the browser extension's permissions/manifest to recognize atcoder.jp domains.
• Add an automated script/handler within the extension to handle form authentication/submission fields on AtCoder's submit pages.
• Integrate with the main CPH VS Code extension so the "Submit" button appears when working on an AtCoder problem.

Additional Context

• AtCoder problems are already parsed easily using the Competitive Companion extension, so adding the submit feature would complete the loop for AtCoder users.
• I'd be happy to test any beta versions or help map out the DOM elements for the AtCoder submission form if needed!