fe_comment: widget v2, overview link setting and inline mail images

- Rebuild the widget server-backed: it renders only when GET state
  allows (logged-in FE user of the configured group), so the script
  tag is safe on public pages; form with mini rich text (bold/italic/
  list), file attachment chips and Ctrl+V image paste - a paste lazily
  creates the draft feedback, uploads and inserts the image inline
- Note pad badge mirrors the server item count; the pad button links
  to the overview page via new extension setting overviewPid, exposed
  as overviewUrl in GET state (slug lookup)
- Embed /fileadmin images of notes into the feedback mail as CID
  parts: relative srcs are dead in mail clients and dev sits behind
  basic auth; attachment links stay absolute URLs
- public/widget-test.html: dev test page, doubles as the reference
  implementation of the React login flow (fetch felogin JSON form
  incl. __RequestToken, urlencoded POST to /_json + form.action)
- Docs section 5x (fe_comment v2 incl. CSRF login lesson,
  templateRootPaths.10 collision, cache:flush before extension:setup)

packages/fe_comment/Classes/Service/ExtensionSettings.php

@@ -68,6 +68,12 @@ final class ExtensionSettings
         return (int)($this->settings['storagePid'] ?? 0);
     }
 
+    /** Page uid of the feedback overview page; 0 = no overview link. */
+    public function overviewPid(): int
+    {
+        return (int)($this->settings['overviewPid'] ?? 0);
+    }
+
     /**
      * Selectable recipients, parsed from "Label <mail>; Label2 <mail2>".
      * The index doubles as the id the widget sends back.