Phase 4 Stage 0: design system foundation (client_app)

- HaloTokens, HaloSpacing, HaloRadius, HaloMotion, HaloShadows (warm palette;
  calm/playful stubbed for phase 5).
- Bundled Bricolage Grotesque, Poppins, JetBrains Mono (~1.2 MB total, OFL).
- haloThemeData() wired into MaterialApp.router with Figma-aligned text
  scale, pill ElevatedButton, 64px input height, 24px-corner BottomSheet,
  dark pill SnackBar.
- Halo* widget primitives: Button, Orb, StepDots, BottomSheet, Popup,
  Snackbar, Chip.
- Dev-only /_theme_preview route gated by --dart-define=THEME_PREVIEW=true
  for visual reference during stages 2-8.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

client_app/assets/fonts/README.md

@@ -0,0 +1,15 @@
+# HaloBestie font assets
+
+Stage 0 design-system fonts. All licensed under the SIL Open Font License.
+
+| File                              | Source |
+|-----------------------------------|--------|
+| `BricolageGrotesque-Variable.ttf` | https://github.com/google/fonts/tree/main/ofl/bricolagegrotesque |
+| `Poppins-Regular.ttf`             | https://github.com/google/fonts/tree/main/ofl/poppins |
+| `Poppins-Medium.ttf`              | https://github.com/google/fonts/tree/main/ofl/poppins |
+| `Poppins-SemiBold.ttf`            | https://github.com/google/fonts/tree/main/ofl/poppins |
+| `Poppins-Bold.ttf`                | https://github.com/google/fonts/tree/main/ofl/poppins |
+| `JetBrainsMono-Variable.ttf`      | https://github.com/google/fonts/tree/main/ofl/jetbrainsmono |
+
+Wired into `client_app/pubspec.yaml` and consumed via `HaloTokens.fontDisplay`,
+`fontBody`, `fontMono` in `client_app/lib/core/theme/halo_tokens.dart`.