refactor: add ChoiceLoader for blog template overrides + update shared_lib

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app.py

@@ -1,7 +1,9 @@
 from __future__ import annotations
 import path_setup  # noqa: F401  # adds shared_lib to sys.path
+from pathlib import Path
 
 from quart import g, request
+from jinja2 import FileSystemLoader, ChoiceLoader
 from sqlalchemy import select
 
 from shared.factory import create_base_app
@@ -49,6 +51,13 @@ async def coop_context() -> dict:
 def create_app() -> "Quart":
     app = create_base_app("coop", context_fn=coop_context)
 
+    # App-specific templates override shared templates
+    app_templates = str(Path(__file__).resolve().parent / "templates")
+    app.jinja_loader = ChoiceLoader([
+        FileSystemLoader(app_templates),
+        app.jinja_loader,
+    ])
+
     # --- blueprints ---
     app.register_blueprint(register_auth_bp())