feat: add Phase 3 Workout UI — auth, profiles, workout viewer, exercise browser

Build the core user-facing experience with admin login (bcrypt + signed
session cookies), profile switcher, workout day viewer with warmups and
exercise cards, and HTMX-powered exercise browser with search/filter.

- AuthService with bcrypt password verification and itsdangerous session tokens
- Auth dependency redirects to /login (303) for unauthenticated requests
- NavContextMiddleware injects admin/profiles/active_profile into all templates
- Profile management (list, switch, edit) with cookie-based active profile
- Workout day viewer shows warmups + exercises + per-user programming targets
- Exercise browser with HTMX filter dropdowns (no page reloads)
- Flash message partial for success/error feedback
- 12 new tests (66 total passing)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pyproject.toml

@@ -6,4 +6,7 @@ requires-python = ">=3.12"
 
 [build-system]
 requires = ["setuptools>=75.0"]
-build-backend = "setuptools.backends._legacy:_Backend"
+build-backend = "setuptools.build_meta"
+
+[tool.setuptools.packages.find]
+include = ["app*"]