F14: add per-activity delete (DELETE /api/activity/{id} + drawer button)

Server endpoint removes the activity JSON, GeoJSON, timeseries, sidecar
edit, and images directory. Also purges the dedup cache entry so the
file can be re-uploaded if needed. Runs merge_all + rebuild afterwards.

EditDrawer: two-click delete button (click once → "Confirm delete?",
click again → deletes). On success, dispatches 'deleted' event.
ActivityDetail navigates back to the feed on delete.

site/src/components/ActivityDetail.svelte

@@ -112,7 +112,7 @@
 <svelte:window on:keydown={onKeydown} />
 
 {#if editOpen && editEnabled}
-  <EditDrawer activityId={activity.id} {editUrl} on:saved={onSaved} on:close={() => editOpen = false} />
+  <EditDrawer activityId={activity.id} {editUrl} on:saved={onSaved} on:close={() => editOpen = false} on:deleted={() => { window.location.href = base; }} />
 {/if}
 
 <!-- Lightbox -->