feat: add "Using the App" guide for iOS app users

Author: felippemsc

docs/guides/using-the-app.md

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+---
+sidebar_position: 4
+---
+
+# Using the App
+
+A step-by-step guide to capturing authenticated photos with the SignedShot iOS app.
+
+## Overview
+
+The SignedShot app captures photos with cryptographic proof of authenticity. Each photo is signed by your device's hardware key and verified by the SignedShot API. The result is two files: the original photo and a sidecar proof that anyone can verify.
+
+## Step 1: Register Your Device
+
+When you first open the app, you'll see a **Device Not Registered** prompt.
+
+1. Tap **Register Device**
+2. Your device will be registered with the SignedShot API
+3. A green shield icon appears in the status bar confirming registration
+
+This is a one-time setup. Your device receives a unique identity used to sign future captures. The registration uses Apple's App Attest to verify that requests come from a genuine device.
+
+:::tip
+If registration fails, check your internet connection and try again. The app will show a **Retry** button on errors.
+:::
+
+## Step 2: Start a Capture Session
+
+Before each photo, you need to start a capture session.
+
+1. Tap **Start Session**
+2. A **Session Active** indicator appears with a countdown timer
+3. The capture button turns white, indicating you can now take a photo
+
+Sessions expire after **5 minutes** as a security measure to prevent replay attacks. If a session expires, tap **Create New Session** to start a fresh one.
+
+:::info
+Each session generates a one-time nonce that ties your capture to a specific moment in time. This ensures the proof can't be reused.
+:::
+
+## Step 3: Capture a Photo
+
+With an active session:
+
+1. Frame your shot using the camera preview
+2. Tap the **capture button** (white circle at the bottom)
+3. Wait for the capture to complete
+
+Behind the scenes, the app:
+- Captures the raw JPEG bytes
+- Signs the content hash with your device's hardware key (Secure Enclave)
+- Exchanges the session nonce for a trust token from the API
+- Generates a sidecar file combining the trust token and media integrity proof
+
+## Step 4: View the Result
+
+After a successful capture, you'll see:
+
+- A thumbnail of your photo
+- The filename (e.g., `photo_20260209_143052.jpg`)
+- **Trust Token Received** confirmation
+- The sidecar filename (e.g., `photo_20260209_143052.sidecar.json`)
+- **Saved to Files → SignedShot** indicating where your files are stored
+
+### Finding Your Files
+
+Photos and sidecar proofs are saved to the **Files** app under the **SignedShot** folder. You can also tap **Open in Files** to jump there directly.
+
+Each capture produces two files:
+
+| File | Description |
+|------|-------------|
+| `photo_YYYYMMDD_HHMMSS.jpg` | The original photo (unmodified JPEG) |
+| `photo_YYYYMMDD_HHMMSS.sidecar.json` | The cryptographic proof |
+
+Both files are needed for verification. The sidecar alone proves nothing — it must be verified against the original media.
+
+## Verifying Your Photo
+
+You can verify a captured photo using the CLI or the API.
+
+### CLI
+
+```bash
+pip install signedshot
+signedshot validate photo_20260209_143052.sidecar.json photo_20260209_143052.jpg
+```
+
+### API
+
+Upload both files to the `/validate` endpoint:
+
+```bash
+curl -X POST https://api.signedshot.io/validate \
+  -F "sidecar=@photo_20260209_143052.sidecar.json" \
+  -F "media=@photo_20260209_143052.jpg"
+```
+
+### What Gets Verified
+
+The validator checks three things:
+
+1. **Capture Trust** — The JWT was signed by the SignedShot API (verified via JWKS)
+2. **Media Integrity** — The SHA-256 hash matches and the device signature is valid
+3. **Cross-Validation** — The capture ID in the JWT matches the one in the media integrity proof
+
+If any check fails, the photo cannot be considered authentic.
+
+## Troubleshooting
+
+### Session Expired
+
+Sessions last 5 minutes. If you see **Session Expired**, tap **Create New Session** and capture again.
+
+### Network Errors
+
+The app requires an internet connection to create sessions and exchange trust tokens. If you see a network error, check your connection and tap **Retry**.
+
+### Unauthorized (401)
+
+If your device session has expired on the server, the app will automatically clear credentials and prompt you to register again. Tap **Register Device** to re-register.
+
+## Next Steps
+
+- [Quick Start](/guides/quick-start) — Verify photos with the Python library or CLI
+- [Sidecar Format](/concepts/sidecar-format) — Understand the proof structure
+- [Two-Layer Trust](/concepts/two-layer-trust) — How Capture Trust and Media Integrity work together