Add opt-in calorie and macro tracking - #96

Summary

Adds opt-in calorie/macro tracking: meal logging via a dedicated Nutrition page (manual entry, Open Food Facts search, barcode scan, AI photo/describe analysis) and via the AI coach (text or photo → log_meal with in-chat meal cards). Nutrition numbers are grounded in Open Food Facts where possible, with per-entry provenance (verified / AI estimate / manual) — never silently guessed. Intake goals (kcal + macros) are set in Settings or by the coach; when enabled, nutrition context flows into coach chat, check-ins, and summaries. Surfaced as an Activity-page card, a Today tile, and a home-screen widget — no new tab. Off by default with granular sub-toggles (share with coach, photo analysis, HealthKit dietary export, Today/widget visibility).

Type of change

• ✨ New feature (non-breaking change that adds functionality)
• 🤖 Coach / LLM change (tools, prompts, orchestration)
• 🎨 UI / DesignSystem change

How was this tested?

• Added / updated unit tests (PulseLoopTests) — 54 new tests (repository, prefs decode, OFF mapper/client with stubbed URLSession, coach tools, goals, context injection)
• Ran the test suite locally — 762/762 pass; app + widget-extension targets build green
• Tested with demo data (-seedDemo YES, no hardware) — verified Activity card, Nutrition page, and settings in the simulator (new -enableNutrition YES / -openNutrition YES test hooks); additive SwiftData migration verified on launch

Privacy & data

• This change does not send health data off-device without explicit user action. Feature is off by default; the only visible trace while disabled is the Settings row. Meal photos reach the user's configured LLM provider only via explicit attach/Analyze actions; Open Food Facts calls are user-triggered (search/scan) and cache-first; HealthKit dietary export is a separate opt-in.
• No secrets, API keys, or personal data are committed.

Checklist

• My code follows the project's style (SwiftLint passes).
• I ran the tests and they pass.