feat(timeline): add a WRITE_ALL-gated edit pencil to WorldBand

A curated HISTORICAL band had no edit affordance at all. Mirror the
EventPill pencil inline at the end of the band (data-testid="event-edit",
/edit href, aria-hidden glyph + sr-only Bearbeiten), gated on
canWrite && eventId != null. Thread canWrite to WorldBand through both
the year-band path and the undated bucket.

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

frontend/src/lib/timeline/YearBand.svelte

@@ -59,7 +59,7 @@ const rows = $derived.by<Row[]>(() => {
 		{#each rows as row (row.t === 'strip' ? `strip-${year.year}` : entryKey(row.entry))}
 			{#if row.t === 'event'}
 				{#if row.entry.type === 'HISTORICAL'}
-					<WorldBand entry={row.entry} />
+					<WorldBand entry={row.entry} canWrite={canWrite} />
 				{:else}
 					<EventPill entry={row.entry} canWrite={canWrite} />
 				{/if}