fix: use sentinel value for optional brood location dropdown

FastHTML omits empty string attributes (value=""), causing browsers to
submit the option's text content "Same as hatch location" instead of an
empty value. This resulted in a ULID validation error.

Use "__none__" as a sentinel value that the server converts to None.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/animaltrack/web/routes/actions.py

@@ -278,7 +278,11 @@ async def hatch_recorded(request: Request, session):
     # Extract form data
     species = form.get("species", "")
     location_id = form.get("location_id", "")
-    assigned_brood_location_id = form.get("assigned_brood_location_id", "") or None
+    # "__none__" is a sentinel value used because FastHTML omits empty string attributes
+    brood_location_raw = form.get("assigned_brood_location_id", "")
+    assigned_brood_location_id = (
+        None if brood_location_raw in ("", "__none__") else brood_location_raw
+    )
     hatched_live_str = form.get("hatched_live", "0")
     notes = form.get("notes", "") or None
     nonce = form.get("nonce")

src/animaltrack/web/templates/actions.py

@@ -327,10 +327,12 @@ def hatch_form(
         )
 
     # Build brood location options (optional)
+    # Note: We use "__none__" as a sentinel value instead of "" because FastHTML
+    # omits empty string attributes, causing browsers to submit the text content.
     brood_location_options = [
         Option(
             "Same as hatch location",
-            value="",
+            value="__none__",
             selected=not selected_brood_location,
         )
     ]