feat(app): allow setting static directory at runtime (#414)

[soniafergusonship]

Summary

Adds a runtime Crow::static_directory() setter/getter so the implicit static file root can be chosen without a compile-time CROW_STATIC_DIRECTORY macro (#414).

• Default remains CROW_STATIC_DIRECTORY ("static/")
• Must be set before run() / run_async() (path is captured when static routes are registered)
• CROW_STATIC_ENDPOINT is unchanged
• Docs updated in docs/guides/static.md

Test plan

• app.static_directory("alt/") serves files from alt/ at the default static endpoint
• Omitting the setter still uses CROW_STATIC_DIRECTORY
• Defining CROW_STATIC_DIRECTORY still sets the default when no runtime call is made

Closes #414