fix(timeline): mark the event form dirty on date/precision/picker changes

The beforeNavigate unsaved-changes guard only fired for title/type/description
(their oninput/onchange hooks set `dirty`). Editing only the date, precision,
end-date, or the linked persons/documents left `dirty` false, so a curator could
navigate away and silently lose those edits — defeating the guard for the senior
author audience. Add an $effect that watches those values and marks dirty on any
change after the initial prop snapshot (first run only arms the watcher).

Addresses PR #832 review (round-2 clean-agent concern).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/timeline/EventForm.svelte

@@ -105,6 +105,22 @@ function markDirty() {
 	dirty = true;
 }
 
+// title/type/description set `dirty` through their own oninput/onchange hooks,
+// but the date/precision/end-date (inside DatePrecisionField) and the two pickers
+// have no such hook — so changing only a date or a linked person would otherwise
+// slip past the beforeNavigate guard. Watch those values and mark dirty on any
+// change after the initial prop snapshot (the first run only arms the watcher).
+let dirtyArmed = $state(false);
+$effect(() => {
+	void dateIso;
+	void precision;
+	void endDateIso;
+	void selectedPersons.length;
+	void selectedDocuments.length;
+	if (dirtyArmed) dirty = true;
+	else dirtyArmed = true;
+});
+
 // Guards a submit with a blank title client-side. The server re-validates and
 // owns the authoritative fail(400) with per-field flags.
 function handleSubmit(e: SubmitEvent) {