feat: extract shared infrastructure from shared_lib

Phase 1-3 of decoupling plan:
- Shared DB, models, infrastructure, browser, config, utils
- Event infrastructure (domain_events outbox, bus, processor)
- Structured logging
- Generic container concept (container_type/container_id)
- Alembic migrations for all schema changes

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

browser/app/utils/htmx.py

@@ -0,0 +1,46 @@
+"""HTMX utilities for detecting and handling HTMX requests."""
+
+from quart import request
+
+
+def is_htmx_request() -> bool:
+    """
+    Check if the current request is an HTMX request.
+
+    Returns:
+        bool: True if HX-Request header is present and true
+    """
+    return request.headers.get("HX-Request", "").lower() == "true"
+
+
+def get_htmx_target() -> str | None:
+    """
+    Get the target element ID from HTMX request headers.
+
+    Returns:
+        str | None: Target element ID or None
+    """
+    return request.headers.get("HX-Target")
+
+
+def get_htmx_trigger() -> str | None:
+    """
+    Get the trigger element ID from HTMX request headers.
+
+    Returns:
+        str | None: Trigger element ID or None
+    """
+    return request.headers.get("HX-Trigger")
+
+
+def should_return_fragment() -> bool:
+    """
+    Determine if we should return a fragment vs full page.
+
+    For HTMX requests, return fragment.
+    For normal requests, return full page.
+
+    Returns:
+        bool: True if fragment should be returned
+    """
+    return is_htmx_request()