Add validation for custom DoH server inputs

[Copilot]

Custom DoH servers could be added without validation, allowing malformed URLs, HTTP URLs, and empty names to reach the capability check.

Changes

• Name validation: Reject empty names after trimming whitespace
• URL format validation: Use URL constructor to catch malformed inputs
• Protocol validation: Enforce HTTPS requirement for DoH
• Input normalization: Trim whitespace, handle commas in URLs via slice(1).join(', ')

// Validate URL format and protocol
let parsedUrl;
try {
    parsedUrl = new URL(url);
} catch (_) {
    alert("Invalid URL format. Please enter a valid URL (e.g., https://example.com/dns-query).");
    return;
}

if (parsedUrl.protocol !== 'https:') {
    alert("DoH URLs must use HTTPS protocol. Please enter a URL starting with 'https://'.");
    return;
}

Validation errors now surface with clear messages before attempting capability checks on invalid endpoints.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Without check for doh url</issue_title>
<issue_description>I use my own doh url, like "https://xxx.net/xxx-query".such as "https://xxx.net/us-query","https://xxx.net/jp-query"</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Without check for doh url #27

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