fix: use SHA-256 for container fingerprints instead of DefaultHasher

DefaultHasher is not stable across Rust compiler versions or binary
rebuilds, causing unnecessary container recreations on every app update.
Replace with SHA-256 for deterministic, cross-build-stable fingerprints.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/src-tauri/Cargo.lock

@@ -4681,6 +4681,7 @@ dependencies = [
  "reqwest 0.12.28",
  "serde",
  "serde_json",
+ "sha2",
  "tar",
  "tauri",
  "tauri-build",