feat(app): cursor encode/decode for paginated search (fb-34)

Opaque base64(JSON{offset, corpus_revision}). Mismatch or
malformed input returns ErrorV1 with code = stale_cursor.
base64 promoted to workspace dep.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -3528,6 +3528,7 @@ name = "kebab-app"
 version = "0.4.0"
 dependencies = [
  "anyhow",
+ "base64 0.22.1",
  "blake3",
  "dirs 5.0.1",
  "ignore",