feat: Update spec, fix bugs, improve UI/UX, and clean up code

FitnessSync/specs/002-fitbit-garmin-sync/plan.md

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+# Implementation Plan: Fitbit/Garmin Data Sync
+
+**Branch**: `002-fitbit-garmin-sync` | **Date**: 2025-12-24 | **Spec**: [link](./spec.md)
+**Input**: Feature specification from `specs/002-fitbit-garmin-sync/spec.md`
+
+## Summary
+
+This feature will implement synchronization of activities and health metrics from Garmin Connect to the application's database. The implementation will involve wiring up existing backend stub endpoints to a sync service that uses the `garth` library to fetch data from Garmin. The synced data will be stored in a PostgreSQL database and exposed through a series of new API endpoints for querying and retrieval.
+
+## Technical Context
+
+**Language/Version**: Python 3.11
+**Primary Dependencies**: FastAPI, Uvicorn, SQLAlchemy, Pydantic, garth
+**Storage**: PostgreSQL
+**Testing**: pytest, pytest-asyncio
+**Target Platform**: Linux server (via Docker)
+**Project Type**: Web application (backend)
+**Performance Goals**: Standard web application performance, with API responses under 500ms p95.
+**Constraints**: The solution should be implemented within the existing `backend` service.
+**Scale/Scope**: The initial implementation will support syncing data for a single user.
+
+## Constitution Check
+
+*GATE: Must pass before Phase 0 research. Re-check after Phase 1 design.*
+
+- **Test-First**: The implementation will follow a test-driven approach. For each endpoint and service function, a corresponding set of unit and/or integration tests will be written before the implementation.
+
+*VERDICT: The plan adheres to the core principles of the constitution.*
+
+## Project Structure
+
+### Documentation (this feature)
+
+```text
+specs/002-fitbit-garmin-sync/
+├── plan.md              # This file
+├── research.md          # Phase 0 output
+├── data-model.md        # Phase 1 output
+├── quickstart.md        # Phase 1 output
+├── contracts/           # Phase 1 output
+│   └── api-contract.yaml
+└── tasks.md             # Phase 2 output (created by /speckit.tasks)
+```
+
+### Source Code (repository root)
+
+The implementation will be contained within the existing `backend` directory structure.
+
+```text
+backend/
+├── src/
+│   ├── api/
+│   │   ├── activities.py
+│   │   ├── metrics.py
+│   │   └── sync.py
+│   ├── models/
+│   │   ├── activity.py
+│   │   └── health_metric.py
+│   └── services/
+│       ├── sync_app.py
+│       └── garmin/
+│           └── client.py
+└── tests/
+    ├── integration/
+    │   ├── test_sync_flow.py
+    └── unit/
+        ├── test_api/
+        │   ├── test_activities.py
+        │   └── test_metrics.py
+        └── test_services/
+            └── test_sync_app.py
+```
+
+**Structure Decision**: The plan adheres to the existing project structure, which is a single backend service. New functionality will be added to the appropriate existing modules.
+
+## Complexity Tracking
+*No constitutional violations to justify.*