fix: problem PID 可能为 null

[boomzero]

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What does this PR aim to accomplish?:

This PR fixes a bug where the ProblemSwitcher utility would fail to display a problem's ID (PID) if the user navigated directly to a contest problem page without first visiting the main contest page.

The utility relied on localStorage being populated by a visit to the contest page. Without this preliminary visit, the necessary data wasn't cached, and the PID would not appear next to the problem title. This PR ensures the PID is always displayed, regardless of the user's navigation path.

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How does this PR accomplish the above?:

• Proactive Data Fetching: The script now checks if the problem's PID is available in localStorage. If it is not found, the script automatically sends a fetch request to the main contest page in the background.
• DOM Parsing & Caching: It then parses the HTML from the fetched contest page, extracts the PIDs for all problems in the contest, and saves them to localStorage.
• Display PID: After the cache is populated, the script retrieves the correct PID for the current problem and appends it to the <h2> title element as intended.
• Refactor: A minor code cleanup was performed, removing an unnecessary intermediate variable (result) to make an assignment more direct.

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