feat(timeline): gate the EventPill edit pencil behind canWrite

Thread a gate-closed canWrite prop through TimelineView -> YearBand ->
EventPill and the undated bucket so a Reader never sees a dead-end edit
link. canEdit now also requires canWrite; the default false keeps an
un-threaded caller closed. The real boundary stays the #781 route guard
plus the backend permission -- this only hides the link.

Refs #842
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/timeline/YearBand.svelte

@@ -16,7 +16,7 @@ type TimelineEntryDTO = components['schemas']['TimelineEntryDTO'];
  * the band holds ≤ 12 (REQ-011) or collapse to a single density strip above that
  * (REQ-012). Entries are never re-sorted — DTO order is preserved (REQ-003).
  */
-let { year }: { year: TimelineYearDTO } = $props();
+let { year, canWrite = false }: { year: TimelineYearDTO; canWrite?: boolean } = $props();
 
 type Row =
 	| { t: 'event'; entry: TimelineEntryDTO }
@@ -61,7 +61,7 @@ const rows = $derived.by<Row[]>(() => {
 				{#if row.entry.type === 'HISTORICAL'}
 					<WorldBand entry={row.entry} />
 				{:else}
-					<EventPill entry={row.entry} />
+					<EventPill entry={row.entry} canWrite={canWrite} />
 				{/if}
 			{:else if row.t === 'letter'}
 				<div class="letter-row" data-side={row.side}>