fix: preserve datetime picker value when form validation fails

The event_datetime_field component now accepts initial_value and
initial_ts parameters. When re-rendering a form after validation
error, the datetime picker value is preserved and shown expanded.

🤖 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

@@ -230,6 +230,10 @@ def _render_cohort_error(
                     selected_sex=form_data.get("sex", "unknown") if form_data else "",
                     selected_origin=form_data.get("origin", "") if form_data else "",
                     count_value=form_data.get("count", "") if form_data else "",
+                    datetime_value=(
+                        form_data.get("cohort_datetime_value", "") if form_data else ""
+                    ),
+                    datetime_ts=form_data.get("ts_utc", "0") if form_data else "0",
                 ),
                 title="Create Cohort - AnimalTrack",
                 active_nav=None,

src/animaltrack/web/templates/actions.py

@@ -26,7 +26,11 @@ from animaltrack.selection.validation import SelectionDiff
 # =============================================================================
 
 
-def event_datetime_field(field_id: str = "event_datetime") -> Div:
+def event_datetime_field(
+    field_id: str = "event_datetime",
+    initial_value: str = "",
+    initial_ts: str = "0",
+) -> Div:
     """Create a collapsible datetime picker for backdating events.
 
     When collapsed, shows "Now" and the event will use the current time.
@@ -35,6 +39,8 @@ def event_datetime_field(field_id: str = "event_datetime") -> Div:
 
     Args:
         field_id: Unique ID prefix for the datetime field elements.
+        initial_value: Initial value for the datetime-local input (for form preservation).
+        initial_ts: Initial value for the hidden ts_utc field (for form preservation).
 
     Returns:
         Div containing the datetime picker with toggle functionality.
@@ -43,6 +49,11 @@ def event_datetime_field(field_id: str = "event_datetime") -> Div:
     picker_id = f"{field_id}_picker"
     input_id = f"{field_id}_input"
 
+    # If initial value is set, start with picker expanded
+    has_initial = bool(initial_value)
+    picker_style = "display: block;" if has_initial else "display: none;"
+    toggle_text = "Use current time" if has_initial else "Set custom date"
+
     # JavaScript for toggle and conversion
     script = f"""
     (function() {{
@@ -84,7 +95,7 @@ def event_datetime_field(field_id: str = "event_datetime") -> Div:
                 Span("Now", cls="text-stone-400"),
                 " - ",
                 Span(
-                    "Set custom date",
+                    toggle_text,
                     id=toggle_id,
                     cls="text-blue-400 hover:text-blue-300 cursor-pointer underline",
                 ),
@@ -95,6 +106,7 @@ def event_datetime_field(field_id: str = "event_datetime") -> Div:
                     id=input_id,
                     name=f"{field_id}_value",
                     type="datetime-local",
+                    value=initial_value,
                     cls="uk-input w-full mt-2",
                 ),
                 P(
@@ -102,11 +114,11 @@ def event_datetime_field(field_id: str = "event_datetime") -> Div:
                     cls="text-xs text-stone-500 mt-1",
                 ),
                 id=picker_id,
-                style="display: none;",
+                style=picker_style,
             ),
             cls="mt-1",
         ),
-        Hidden(name="ts_utc", value="0"),
+        Hidden(name="ts_utc", value=initial_ts),
         Script(script),
         cls="space-y-1",
     )
@@ -127,6 +139,8 @@ def cohort_form(
     selected_sex: str = "unknown",
     selected_origin: str = "",
     count_value: str = "",
+    datetime_value: str = "",
+    datetime_ts: str = "0",
     action: Callable[..., Any] | str = "/actions/animal-cohort",
 ) -> Form:
     """Create the Cohort Creation form.
@@ -141,6 +155,8 @@ def cohort_form(
         selected_sex: Pre-selected sex.
         selected_origin: Pre-selected origin.
         count_value: Pre-filled count value.
+        datetime_value: Pre-filled datetime-local input value.
+        datetime_ts: Pre-filled ts_utc hidden field value.
         action: Route function or URL string for form submission.
 
     Returns:
@@ -253,7 +269,7 @@ def cohort_form(
             name="origin",
         ),
         # Event datetime picker (for backdating)
-        event_datetime_field("cohort_datetime"),
+        event_datetime_field("cohort_datetime", datetime_value, datetime_ts),
         # Hidden nonce for idempotency
         Hidden(name="nonce", value=str(ULID())),
         # Submit button