Add uncompressed-size and new-chunk-size columns to snapshot list

The remote snapshot table now shows the total plaintext size of all
chunks referenced by each snapshot, plus the plaintext size of chunks
newly referenced by that snapshot (chunks not in any earlier completed
snapshot known to the local DB). The latter is the marginal data
introduced by each backup — useful for spotting which snapshots
actually added bytes vs. dedup'd against prior state.

Both new columns are computed from the local database only. Snapshots
that exist in remote storage but not in the local DB show
"<remote only>" in those cells; their COMPRESSED SIZE column still
reflects the value fetched from the remote manifest.

internal/vaultik/helpers.go

@@ -10,11 +10,17 @@ import (
 	"sneak.berlin/go/vaultik/internal/types"
 )
 
-// SnapshotInfo contains information about a snapshot
+// SnapshotInfo contains information about a snapshot.
+// UncompressedSize and NewChunkSize are populated only when the snapshot
+// is present in the local database; LocallyTracked indicates whether
+// those values are meaningful.
 type SnapshotInfo struct {
-	ID             types.SnapshotID `json:"id"`
-	Timestamp      time.Time        `json:"timestamp"`
-	CompressedSize int64            `json:"compressed_size"`
+	ID               types.SnapshotID `json:"id"`
+	Timestamp        time.Time        `json:"timestamp"`
+	CompressedSize   int64            `json:"compressed_size"`
+	UncompressedSize int64            `json:"uncompressed_size,omitempty"`
+	NewChunkSize     int64            `json:"new_chunk_size,omitempty"`
+	LocallyTracked   bool             `json:"locally_tracked"`
 }
 
 // formatBytes formats bytes in a human-readable format