Minor: Missing runtime dependency "addict" on fresh editable install, DA3 import fails

# Bug: missing `addict` dependency (fresh install)

On a clean install from `main`, `DepthAnything3` fails to import because `addict` isn’t installed, even though DA3 imports `addict.Dict` at runtime.

**Env**
- Windows 10 Pro 19045 (build 19045)
- RTX 3090, driver 572.16 (CUDA 12.8 supported)
- CUDA toolkit 11.2
- Python 3.10.19 (conda env `da3`)
- Conda 24.1.2
- pip 25.3
- DA3 tested on `main` @ `ed6989a23cd389e975ed9f7cbd7385396e6d867e`

**Reproduction**
```powershell
conda create -n da3 python=3.10 -y
conda activate da3
git clone https://github.com/ByteDance-Seed/Depth-Anything-3.git
cd Depth-Anything-3
pip install -e .
pip install -r requirements.txt
python -c "from depth_anything_3.api import DepthAnything3"
```

**Error**
```
ModuleNotFoundError: No module named 'addict'
from depth_anything_3/model/da3.py: from addict import Dict
```

**Workaround**
```powershell
pip install addict
```

**Expected fix**
Editable install / `requirements.txt` should include `addict` so imports work out‑of‑the‑box.
Fix: add `addict` to `requirements.txt` and to the install metadata (`pyproject.toml` dependencies).