feat: Implement Analyze and Plan features, Smart Sync, Theming, and Optional Auth

docs/garmin_login.md

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+# Garmin Connect Login Setup
+
+To sync your fitness data from Garmin Connect, you need to provide your credentials.
+
+## Credentials Configuration
+
+Create a `.env` file in the `backend/` directory with the following content:
+
+```env
+GARMIN_EMAIL=your_email@example.com
+GARMIN_PASSWORD=your_password
+```
+
+> [!IMPORTANT]
+> If you have Multi-Factor Authentication (MFA) enabled on your Garmin account, the integration will prompt you for the code during the first run. The session will be saved locally in `backend/.garth` to avoid repeated logins.
+
+## Security Note
+
+- The `.env` file is included in `.gitignore` and will never be committed.
+- Your credentials are only used locally to authenticate with Garmin's official login service via the `garth` library.

docs/user_manual.md

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+# User Manual: Fitness Antigravity
+
+Fitness Antigravity is a tool for managing your fitness data locally and getting AI-powered recommendations.
+
+## Core Concepts
+
+1. **Local Storage**: All data synced from Garmin or Withings is stored locally in `data/local/` as JSON files.
+2. **Manual Sync**: You control when your data is updated by running the provided scripts.
+3. **AI Recommendations**: Gemini analyzes your local history to suggest your next training focus.
+
+## How to Use
+
+### 1. Syncing Garmin Data
+To download your latest workouts:
+```bash
+cd backend
+python scripts/sync_garmin.py
+```
+*Note: Ensure your `.env` is configured (see [Garmin Login](garmin_login.md)).*
+
+### 2. Creating Local Workouts
+To design a strength training session:
+```bash
+cd backend
+python scripts/create_workout.py
+```
+Follow the prompts to add exercises and reps. The workout will be saved in `data/local/workouts/`.
+
+### 3. Getting Recommendations
+To get advice on your next session:
+```bash
+cd backend
+python scripts/recommend.py
+```
+
+### 4. Visualizing Data
+To see your progress in the browser:
+```bash
+cd frontend
+npm run dev
+```
+Open the provided URL (usually `http://localhost:5173`) to view the dashboard.
+
+## Objectives
+Your current objective is: **Trained, strong and endurance.** Gemini will tailor all recommendations to this goal.

docs/withings_login.md

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+# Withings Integration Setup
+
+*Note: Withings integration is planned for a future update.*
+
+## Requirements
+To gather information from your Withings account, you will need:
+1. A Withings Developer account.
+2. A Client ID and Client Secret.
+
+## Configuration
+When implemented, you will add these to your `.env`:
+```env
+WITHINGS_CLIENT_ID=your_id
+WITHINGS_CLIENT_SECRET=your_secret
+```
+
+## Data Sync
+Withings data (weightings) will be stored in `data/local/withings/`.