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.

backend/internal/store/reset.go

@@ -0,0 +1,27 @@
+package store
+
+import (
+	"context"
+	"fmt"
+)
+
+// ResetAllSyncedData deletes every synced activity (cascading to its laps,
+// activity_samples, and kind_assignments via ON DELETE CASCADE) and rewinds
+// the backfill watermark to its initial state, so a subsequent Backfill
+// starts a genuinely fresh pull instead of thinking history is already
+// covered. Workout kinds (the user's taxonomy) are left untouched.
+func (db *DB) ResetAllSyncedData(ctx context.Context) error {
+	tx, err := db.BeginTx(ctx, nil)
+	if err != nil {
+		return fmt.Errorf("begin reset tx: %w", err)
+	}
+	defer tx.Rollback()
+
+	if _, err := tx.ExecContext(ctx, `DELETE FROM activities`); err != nil {
+		return fmt.Errorf("delete activities: %w", err)
+	}
+	if _, err := tx.ExecContext(ctx, `UPDATE sync_state SET earliest_synced_date = NULL, backfill_complete = 0 WHERE id = 1`); err != nil {
+		return fmt.Errorf("reset sync state: %w", err)
+	}
+	return tx.Commit()
+}