Add devcontainer with Python 3.13 and uv support

Adds a Dockerfile-based development container using Microsoft's Python 3.13 image, with uv installed for dependency management.

Updates:
- Adds .devcontainer/Dockerfile based on Python 3.13
- Modifies devcontainer.json to use a local build
- Updates on-create-command.sh to install dependencies using uv

.devcontainer/Dockerfile

@@ -0,0 +1,9 @@
+# Base image: Match with .readthedocs.yaml and pyproject.toml
+FROM mcr.microsoft.com/devcontainers/python:3.13
+
+# Install uv and pre-commit globally
+RUN pip install --upgrade pip setuptools wheel uv pre-commit
+
+# Set workdir and default shell
+WORKDIR /workspaces/flask
+SHELL ["/bin/bash", "-c"]