Silence prune and snapshot removal status under --cron and --quiet (closes #294)

PruneBlobs, CleanupLocalSnapshots, RemoveSnapshot, RemoveAllSnapshots
and snapshot purge wrote their status lines straight to stdout, so
`snapshot create --cron --prune` printed on every successful run and
--quiet did not silence `prune`, `snapshot purge`, `snapshot remove` or
`remote nuke`. Those lines now go through internal/ui, which drops them
under --cron and --quiet; warnings, errors and --json documents still
print.

The confirmation prompts still write straight to stdout, as the
`database delete` prompt does, together with what they ask to delete:
the snapshot list before the purge prompt, and the blob count and size
before the prune prompt. With --force those lines are status lines.

The two purge test fixtures gained the UI writer production always sets.

Model: opus-5-5

internal/vaultik/purge_local_remote_consistency_test.go

@@ -14,6 +14,7 @@ import (
 	"sneak.berlin/go/vaultik/internal/log"
 	"sneak.berlin/go/vaultik/internal/snapshot"
 	"sneak.berlin/go/vaultik/internal/types"
+	"sneak.berlin/go/vaultik/internal/ui"
 	"sneak.berlin/go/vaultik/internal/vaultik"
 )
 
@@ -58,13 +59,16 @@ func setupConsistencyTest(
 		require.NoError(t, err)
 	}
 
+	stdout := &bytes.Buffer{}
+
 	v := &vaultik.Vaultik{
 		Storage:      mockStorage,
 		Repositories: repos,
 		DB:           db,
-		Stdout:       &bytes.Buffer{},
+		Stdout:       stdout,
 		Stderr:       &bytes.Buffer{},
 		Stdin:        &bytes.Buffer{},
+		UI:           ui.NewWithColor(stdout, false),
 	}
 	v.SetContext(ctx)