fix(admin): add missing URL field in server edit modal

[ecthelion77]

🐛 Bug-fix PR

Fixes #4492

📌 Summary

The server edit modal is missing the URL input field that servers.js references via safeGetElement('edit-server-url'). The create server modal has the URL field, but the edit modal does not — making it impossible to view or modify a server's URL after creation.

Likely a regression from PR #337 (July 2025) which restructured the server modals.

🔁 Reproduction Steps

1. Navigate to Virtual MCP Servers in admin UI
2. Click edit on any server
3. Edit modal opens without a URL field
4. No way to view or change the URL

See issue #4492 for details.

🐞 Root Cause

The <input id="edit-server-url"> element was never added to the server-edit-modal form in admin.html. The JavaScript populates it via safeGetElement('edit-server-url') which silently returns null, so the URL is lost on save.

💡 Fix Description

Add the URL input field (type="url", id="edit-server-url") to the server edit modal, between the Name and Description fields, matching the create modal pattern:

<div class="relative">
  <label class="block text-sm font-medium text-gray-700 dark:text-gray-300">URL</label>
  <input type="url" name="url" required id="edit-server-url"
    class="mt-1 px-3 py-2 block w-full rounded-md border border-gray-600 shadow-sm ..." />
  <p data-error-message-for="url" class="...">URL is required.</p>
</div>

Single file change: mcpgateway/templates/admin.html (+14 lines).

🧪 Verification

Check	Command	Status
Lint suite	make lint	✅
Unit tests	make test	✅
Manual regression no longer fails	Verified in browser	✅

📐 MCP Compliance

• No impact on MCP protocol
• No breaking change to MCP clients

✅ Checklist

• Code formatted
• No secrets/credentials committed
• Signed-off-by included in commit