fix(app): keep blueprints when CROW_DISABLE_STATIC_DIR is set (#941)

[soniafergusonship]

Summary

CROW_DISABLE_STATIC_DIR gated both add_blueprint() and add_static_dir(), so blueprint routes were never registered (router_.validate_bp() never ran).

This moves blueprint validation into Crow::validate() so blueprints stay active when the static-directory define is set. Global/blueprint static file routes remain behind CROW_DISABLE_STATIC_DIR.

Test plan

• Build with CROW_DISABLE_STATIC_DIR and run examples/example_blueprint.cpp — blueprint routes respond
• app.debug_print() shows blueprint routes when the define is set
• Without the define, default /static/ behavior is unchanged
• Existing blueprint unit test still passes

Fixes #941

[gittiver]

shoudn't this be added to the initial PR?