Health page: VO2 max gauge, layout improvements, 3Y/5Y trends, biological sex profile

- Add biological_sex field to User model, profile API, and ProfilePage toggle (male/female) — used to select the correct ACSM VO2 max threshold table
- Replace simple VO2 max number in daily snapshot with a colour-coded SVG radial gauge (Very Poor=red, Poor=orange, Fair=green, Good=blue, Excellent=purple) driven by sex- and age-appropriate thresholds
- Shrink Sleep widget to half-width, expand Heart & HRV to half-width; reorganise Heart & HRV internals into a 2×2 grid to reduce vertical height
- Add connectNulls + showDots to VO2 Max trend chart so sparse readings connect with a continuous line
- Add 3Y and 5Y range options to the Trends selector; increase allDays limit to 2000 for full 5yr snapshot navigation

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

backend/app/models/user.py

@@ -27,6 +27,7 @@ class User(Base):
     resting_heart_rate = Column(Integer, nullable=True)
     birth_year = Column(Integer, nullable=True)
     height_cm = Column(Float, nullable=True)
+    biological_sex = Column(String(8), nullable=True)   # 'male' | 'female'
 
     # PocketID config (stored per-user so admin can set via UI)
     pocketid_issuer = Column(String(512), nullable=True)