feat: bundle Luciole, Literata, Charis SIL and a Hebrew fallback face

app/lib/application/fonts.dart

@@ -0,0 +1,60 @@
+/// The typefaces bundled with the client (spec 8.2: the reader chooses).
+/// Pure data, shared by the settings page that offers them and the renderer
+/// that applies them.
+library;
+
+/// One bundled family: the name used in pubspec.yaml and stored in settings,
+/// the id of its display name in the locale file, and where to look when it
+/// has no glyph for a character.
+typedef ReaderFont = ({String family, String labelId, List<String> fallback});
+
+/// Offered in this order: the two sans faces first, then the two serifs.
+/// Adding a font is four files in assets/fonts, one `fonts` entry in
+/// pubspec.yaml, one entry here, one label in the locale file, and its licence
+/// beside the files.
+///
+/// Each face names where to look for a character it has no glyph for. Hebrew
+/// comes from a bundled Hebrew face, matched to the shape of the chosen one,
+/// so it never depends on what the device happens to have installed. The
+/// serifs also cover Greek, Cyrillic and a wide Latin range, so each falls
+/// back to the other; the sans faces are narrow and have no bundled sans to
+/// turn to, so for those the platform answers, and it answers a sans with a
+/// sans. Scripts nothing here covers, such as Arabic and the CJK languages,
+/// always come from the platform.
+const readerFonts = <ReaderFont>[
+  (
+    family: 'Atkinson Hyperlegible',
+    labelId: 'settings-font-atkinson',
+    fallback: [hebrewSans],
+  ),
+  (
+    family: 'Luciole',
+    labelId: 'settings-font-luciole',
+    fallback: [hebrewSans],
+  ),
+  (
+    family: 'Literata',
+    labelId: 'settings-font-literata',
+    fallback: ['Charis SIL', hebrewSerif],
+  ),
+  (
+    family: 'Charis SIL',
+    labelId: 'settings-font-charis',
+    fallback: ['Literata', hebrewSerif],
+  ),
+];
+
+/// Bundled to be fallen back on, never offered: the reader picks a face for
+/// the Latin text they are reading, and Hebrew follows its shape.
+const hebrewSans = 'Noto Sans Hebrew';
+const hebrewSerif = 'Noto Serif Hebrew';
+
+const defaultFontFamily = 'Atkinson Hyperlegible';
+
+/// Where to look after [family], or nothing when it is not one of ours.
+List<String> fallbackFor(String family) {
+  for (final font in readerFonts) {
+    if (font.family == family) return font.fallback;
+  }
+  return const [];
+}