bundle Josefin Sans as the default UI face at weight 300

Vendor the JosefinSans variable font (upright + italic, weight range
100-700), OFL licensed. _AppRoot installs it as the ambient
DefaultTextStyle at weight w300 (Light); ClideText inherits rather
than re-specifying the family, so Alchemist's Ahem injection keeps
goldens deterministic per D-024 while production renders the
intended face.

typography.dart gains clideUiFamily + clideUiFamilyFallback
alongside the existing mono constants; .gitignore learns about
Alchemist's failures/ diff output dir so local re-runs don't leak
into `git status`.

Co-Authored-By: Claude <noreply@anthropic.com>

app/lib/widgets/src/typography.dart

@@ -1,12 +1,48 @@
 /// Typography constants shared across widgets.
 ///
-/// Terminal panes, diff views, and any other monospace surface should
-/// import from here rather than hardcoding a family. The fallback chain
-/// exists for web builds + platforms where the bundled `JetBrainsMono`
+/// Two bundled families:
+///
+/// - [clideUiFamily] — Josefin Sans, the application-wide UI face.
+///   Shipped as a variable font (weights 100-700) + italic companion;
+///   default weight is [clideUiDefaultWeight] (Light / `w300`).
+/// - [clideMonoFamily] — JetBrains Mono, for terminal panes, diff
+///   views, code editors, and any other monospace surface.
+///
+/// Fallback chains exist for web builds + harnesses where the bundled
 /// asset isn't picked up (rare, but possible during `flutter test` if
-/// assets aren't declared in the harness).
+/// asset fonts aren't declared in the harness).
 library;
 
+import 'package:flutter/widgets.dart' show FontWeight;
+
+// ---------------------------------------------------------------------------
+// UI face — Josefin Sans
+// ---------------------------------------------------------------------------
+
+/// The bundled application UI family. Always resolved first.
+const String clideUiFamily = 'JosefinSans';
+
+/// Default weight for UI text. Josefin Sans reads well at Light; the
+/// rest of the design adjusts contrast and size to stay legible.
+const FontWeight clideUiDefaultWeight = FontWeight.w300;
+
+/// System fallback chain for the UI face. Sans-serif humanist faces
+/// that sit close to Josefin's proportions, ordered by platform.
+const List<String> clideUiFamilyFallback = [
+  // User system install of Josefin, if any.
+  'Josefin Sans',
+  // Platform humanist sans defaults.
+  'Inter',
+  'Helvetica Neue',
+  'Helvetica',
+  'Arial',
+  'sans-serif',
+];
+
+// ---------------------------------------------------------------------------
+// Monospace face — JetBrains Mono
+// ---------------------------------------------------------------------------
+
 /// The bundled monospace family. Always resolved first.
 const String clideMonoFamily = 'JetBrainsMono';
 
@@ -17,9 +53,8 @@ const List<String> clideMonoFamilyFallback = [
   'SF Mono',
   'Menlo',
   'Monaco',
-  // Linux
-  'JetBrains Mono',  // if the user has it system-installed under the
-                     // canonical PostScript name
+  // Linux — user system install under the canonical PostScript name
+  'JetBrains Mono',
   'Fira Code',
   'Hack',
   'DejaVu Sans Mono',