Merge backfill into Sync now, replace Full backfill with a destructive Reset all

Sync now now runs Backfill (resumes from the watermark, so a widened history
horizon is picked up automatically) then IncrementalSync then detail-fill, in
one click. "Full backfill" is gone -- in its place, "Reset all" (styled as a
destructive action, gated by a confirm dialog) wipes every synced activity
and its laps/kind assignments and rewinds the backfill watermark, so the next
sync performs a genuinely fresh pull instead of trying to patch up existing
rows with newer schema fields.

frontend/src/api/client.ts

@@ -36,7 +36,10 @@ export const api = {
 
   // Sync
   syncRun: () => request<{ status: string }>("/api/sync/run", { method: "POST" }),
-  syncBackfill: () => request<{ status: string }>("/api/sync/backfill", { method: "POST" }),
+  // Deletes every synced activity (and its laps/samples/kind assignments)
+  // and rewinds the backfill watermark -- destructive, gated by a
+  // confirmation in the UI.
+  resetSync: () => request<{ status: string }>("/api/sync/reset", { method: "POST" }),
   syncRuns: () => request<SyncRun[]>("/api/sync/runs"),
   syncStatus: () => request<SyncStatus>("/api/sync/status"),