Fixes #20256

What this fixes

On all preferences pages (/preferences/features, /preferences/sql, /preferences/navigation, /preferences/main-panel, etc.) the active sub-tab was not remembered after a page refresh or after clicking Apply — the page always reset to the first tab.

Root cause

This is a regression that affects both 5.2 and 6.0.

Commit 53f5247 ("Merge FormDisplay::getDisplay templates") replaced the old custom ul.tabs jQuery tab system with Bootstrap data-bs-toggle="tab". The old implementation in js/config.js had:

• setTab() — set location.hash = 'tab_' + tabId and wrote the value into input[name=tab_hash]
• setupConfigTabs() — wired up tab click handlers
• window.onhashchange — read location.hash on page load and activated the matching tab

When Bootstrap tabs were introduced, all three were deleted without a replacement, even though the PHP side was never broken: MainPanelController (and all other preferences controllers) still read tab_hash from the POST body and redirect to the same URL with the tab ID as a URL fragment via UserPreferences::getUrlToRedirect().

Fix

Restore the behaviour using Bootstrap's own API in js/src/config.js:

1. On page load — read window.location.hash and activate the matching tab via bootstrap.Tab.getOrCreateInstance().
2. On shown.bs.tab — write the tab id back to window.location.hash and to input[name=tab_hash], so the PHP redirect after Apply also lands on the correct tab.
3. On teardown — remove the shown.bs.tab listener to avoid leaks across AJAX navigation.

Test

Added a Selenium E2E test testActiveTabRestoredAfterRefresh in ServerSettingsTest that:

1. Navigates to Main panel preferences
2. Switches to the "Tabs" sub-tab
3. Reloads the page
4. Asserts that "Tabs" is still active and "Startup" is not visible