feat(learn): + opens a sheet as Claude's, Camera, Photos and Files, and what goes along in rounded rows

Following the developer's review of 3 Nov 2026 (ADR 0021, D67):

- + below the assistant's field, and below a conversation's follow-up,
  opens "Add to your question" as Claude's "Add to session": three big
  tiles side by side, Camera (where the device takes photos) and Photos
  for an assistant reading images, and Files; below them, in the field, a
  row leading to what goes along, showing how far the assistant looks and
  how many parts are chosen.
- Camera and Photos through image_picker (flutter.dev's own): the camera's
  own app and the system's photo picker, no permission asked on Android, a
  file dialog on the desktop, which has no Camera; photos made at most
  2048 pixels a side, read on the device like any file attached.
- What goes along is chosen in rounded rows, as Claude's Permission, in
  place of radio buttons and chips: how far it looks with a tick, private
  studysets with a switch, the parts each with a tick, a card's image with
  its thumbnail.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/learn/halkyon_learn_flutter/lib/l10n/app_de.arb

@@ -2131,5 +2131,9 @@
   "studysetPrivacyTopic": "Seine Privatsphäre ist die des Lernsets, in dem es liegt.",
   "studysetPrivacyPublish": "Veröffentlichen …",
   "studysetSheetVisibility": "Sichtbarkeit",
-  "studysetSheetPrivate": "Privat"
+  "studysetSheetPrivate": "Privat",
+  "assistantAddTitle": "Zu deiner Frage hinzufügen",
+  "assistantAddCamera": "Kamera",
+  "assistantAddPhotos": "Fotos",
+  "assistantAddFiles": "Dateien"
 }