feat(editor): links, and copying and pasting with every format

Text gets a link from the format bar's new link button or with Ctrl+K
(⌘K): the address for the selection, an empty one taking it away, or at
a caret the text as well, written there linked. An address without its
scheme leads to the web. This replaces the editor's own link menu, which
had no button and spoke English only.

Copying and pasting between the app's editors keeps every format: bold,
links, furigana, formulas, callouts, spoilers. The system's clipboard
holds plain text only (the editor's own copy drops the HTML it makes),
so the blocks copied last stay in memory beside it, and a paste takes
them while the clipboard still holds their text; anything else pastes as
plain text, as before, and other apps get plain text. On phones, which
had no way to copy at all, a bar over selected text copies, cuts, pastes
and selects all (the editor's MobileFloatingToolbar), never over a block
taken whole, whose own buttons a tap is for.

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_localizations_en.dart

@@ -2726,6 +2726,27 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get formatRuby => 'Furigana (a reading above the text)';
 
+  @override
+  String get formatLink => 'Link';
+
+  @override
+  String get linkTitle => 'Link';
+
+  @override
+  String get linkText => 'Text';
+
+  @override
+  String get linkAddress => 'Address';
+
+  @override
+  String get linkAddressHint => 'https://…';
+
+  @override
+  String get linkSave => 'Save';
+
+  @override
+  String get linkRemove => 'Remove link';
+
   @override
   String get rubyTitle => 'Furigana';