feat(editor): text above is named by the language, shown on hover or a long press

Furigana was the only name for text set above text, though it carries
pinyin, a pronunciation or a translation as well. Its button and dialog
now say furigana in a Japanese studyset, pinyin in a Chinese one, and
text above elsewhere, each with an example of its own: the studyset's
languages reach the core's editors through ContentLanguages, which the
studyset's language scope now sets.

Read-only, the text above shows only under the pointer or after a long
press (another hides it), so the text can be read without it first; its
room stays, so no line moves. While editing, it shows over the pointer
above its dotted text; with the caret inside such text, the button
selects all of it and edits what it has, so it can be seen and changed
without finding its ends.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMm7usAo6q9ZZRoPkWjoxi
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/lib/l10n/app_de.arb

@@ -790,7 +790,9 @@
   "formatUnderline": "Unterstrichen",
   "formatSuperscript": "Hochgestellt",
   "formatSubscript": "Tiefgestellt",
-  "formatRuby": "Furigana (eine Lesung über dem Text)",
+  "formatRuby": "Text darüber (Aussprache, Übersetzung)",
+  "formatRubyFurigana": "Furigana (eine Lesung über dem Text)",
+  "formatRubyPinyin": "Pinyin (über den Schriftzeichen)",
   "formatLink": "Link",
   "linkTitle": "Link",
   "linkText": "Text",
@@ -798,7 +800,12 @@
   "linkAddressHint": "https://…",
   "linkSave": "Speichern",
   "linkRemove": "Link entfernen",
-  "rubyTitle": "Furigana",
+  "rubyTitle": "Text darüber",
+  "rubyTitleFurigana": "Furigana",
+  "rubyTitlePinyin": "Pinyin",
+  "rubyText": "Text",
+  "rubyHint": "z. B. eine Aussprache oder eine Übersetzung",
+  "rubyHintPinyin": "Mit Tonzeichen, z. B. hànzì",
   "rubyReading": "Lesung",
   "rubyReadingHint": "In Kana, z. B. かんじ",
   "rubySave": "Speichern",