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/ext_conf_template.txt

@@ -24,3 +24,6 @@ recipients =
 
 # cat=basic//20; type=string; label=Storage pid: page/folder uid the feedback records are stored on (backend visibility). 0 = root.
 storagePid = 0
+
+# cat=basic//30; type=string; label=Overview page (uid): page showing the feedback overview; its URL is delivered to the widget as the note pad link. 0 = no link.
+overviewPid = 0