Use Docker for local development environment

- Update README and CLAUDE.md with Docker-based dev workflow
- Add Dockerfile for containerized deployment
- Add gunicorn to requirements.txt
- Dev uses volume mount for live code reloading

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

README.md

@@ -4,14 +4,7 @@ A spending tracker web application built with Flask.
 
 ## Setup
 
-### 1. Install Dependencies
-
-```bash
-cd src
-pip install -r requirements.txt
-```
-
-### 2. Create Environment Files
+### 1. Create Environment Files
 
 Copy the example file and configure for your environment:
 
@@ -33,17 +26,22 @@ Generate secure keys with:
 python -c "import secrets; print(secrets.token_hex(32))"
 ```
 
+### 2. Build Docker Image
+
+```bash
+docker build -t kebuu .
+```
+
 ### 3. Run the Application
 
 **Development:**
 ```bash
-cd src
-python app.py
+docker run -p 5000:5000 --env-file .env.dev -v $(pwd)/src:/app kebuu
 ```
+The volume mount (`-v`) enables live code reloading during development.
 
-**Production (Docker):**
+**Production:**
 ```bash
-docker build -t kebuu .
 docker run -p 5000:5000 --env-file .env.prod kebuu
 ```