refactor: simplify SubmitLink selection by querying all anchors and finding by text

[boomzero]

What does this PR aim to accomplish?:

Simplify and stabilize how we select the Submit link on the problem page.
Previously we relied on a brittle chain of nth-child() selectors with multiple null checks, which repeatedly broke whenever the DOM structure changed (see #869, #861).
This PR queries all anchors within the container and selects the one whose text is '提交', making the code resilient to DOM reordering and easier to maintain.

How does this PR accomplish the above?:

• Replace multiple positional selectors and if (SubmitLink == null) fallbacks with a single, readable selection:

  const links = document.querySelectorAll('.mt-3 > center:nth-child(1) > a');
  const SubmitLink = Array.from(links).find(a => a.textContent.trim() === '提交');

• Benefits:

  • Removes brittle nth-child() dependencies
  • Reduces code duplication and complexity
  • Improves readability and maintainability

No UI/behavior changes expected; logic is functionally equivalent with a more robust selector.

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