show a friendly page for dead share links, drop plugin-specific selector defaults

Opening an expired or invalid link used to dump raw problem-details
JSON at the guest. Now they get a small page matching the sign-in
look, with the same neutral wording whether the link expired or never
existed (so tokens cannot be probed by outsiders), and an automatic
redirect to the Jellyfin home page. Fixes #1.

GuestHiddenSelectors now defaults to empty instead of shipping CSS
selectors for a plugin nobody else runs. Existing installs keep their
saved value. Fixes #3.

Jellyfin.Plugin.ShareLinks/Web/configPage.html

@@ -125,7 +125,7 @@
 
                         <div class="sl-field inputContainer" style="grid-column: 1 / -1;">
                             <input is="emby-input" type="text" id="GuestHiddenSelectors" label="Guest hidden selectors (CSS, comma-separated)" />
-                            <div class="fieldDescription">Elements hidden from guest sessions, e.g. other plugins' buttons. The default hides a plugin I had installed that injects its own floating UI into the web client.</div>
+                            <div class="fieldDescription">Elements hidden from guest sessions, e.g. other plugins' buttons. Empty by default.</div>
                         </div>
 
                         <div class="sl-field inputContainer">
@@ -212,7 +212,7 @@
                     page.querySelector('#MaxExpiryHours').value = cfg.MaxExpiryHours || 720;
                     page.querySelector('#PublicBaseUrlOverride').value = cfg.PublicBaseUrlOverride || '';
                     page.querySelector('#GuestUsernamePrefix').value = cfg.GuestUsernamePrefix || 'share-';
-                    page.querySelector('#GuestHiddenSelectors').value = cfg.GuestHiddenSelectors != null ? cfg.GuestHiddenSelectors : '.ais-fab,.ais-overlay,#ais-styles';
+                    page.querySelector('#GuestHiddenSelectors').value = cfg.GuestHiddenSelectors != null ? cfg.GuestHiddenSelectors : '';
                     page.querySelector('#AllowTranscoding').checked = cfg.AllowTranscoding !== false;
                     page.querySelector('#AllowRemuxing').checked = cfg.AllowRemuxing !== false;
                     page.querySelector('#CleanupIntervalMinutes').value = cfg.CleanupIntervalMinutes || 60;