fix(timeline): track form dirtiness via change callbacks, not an $effect

The round-2 dirty-guard used an $effect that both read and wrote its `dirtyArmed`
$state, so the self-write re-triggered the effect and `dirty` flipped true on
mount — the beforeNavigate confirm then fired on every navigation away from an
untouched form (caught by the round-3 clean-agent review + the Svelte autofixer,
which flags assigning state inside $effect).

Replace it with the component's existing idiomatic pattern: DatePrecisionField,
PersonMultiSelect, and DocumentMultiSelect each gain an optional `onchange`
callback fired on a real user edit, and EventForm passes `markDirty` to all
three. Now date/precision/end-date and picker add/remove mark the form dirty
exactly like title/type/description already did — no effect, no mount-timing
trap. The new props are optional, so the other consumers (WhoWhenSection, the
document forms) are unaffected. Svelte autofixer: clean.

Addresses PR #832 review (round-3 clean-agent concern).

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

frontend/src/lib/timeline/EventForm.svelte

@@ -101,26 +101,15 @@ beforeNavigate(({ cancel }) => {
 	}
 });
 
+// Every editable control routes its change through markDirty so the
+// beforeNavigate guard catches edits to the date/precision/end-date and the
+// pickers too — not just title/type/description (their onchange callbacks call
+// this). No $effect: marking dirty from the actual edit events avoids a
+// snapshot-vs-effect mount-timing trap.
 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) {
@@ -238,6 +227,7 @@ async function confirmDelete(e: SubmitEvent) {
 							precisionInputName="precision"
 							dateLabel={m.form_label_date()}
 							dateError={dateError}
+							onchange={markDirty}
 							dateTestId="event-date"
 							precisionTestId="event-precision"
 							endDateInnerTestId="event-end-date"
@@ -279,6 +269,7 @@ async function confirmDelete(e: SubmitEvent) {
 						inputId="event-persons-input"
 						hiddenInputName="personIds"
 						emptyLabel={m.event_editor_persons_empty()}
+						onchange={markDirty}
 					/>
 				</div>
 
@@ -295,6 +286,7 @@ async function confirmDelete(e: SubmitEvent) {
 						inputId="event-documents-input"
 						hiddenInputName="documentIds"
 						emptyLabel={m.event_editor_documents_empty()}
+						onchange={markDirty}
 					/>
 				</div>
 			</div>