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.toml

@@ -30,6 +30,7 @@ fern = { version = "0.7", features = ["date-based"] }
 tar = "0.4"
 reqwest = { version = "0.12", default-features = false, features = ["json", "rustls-tls"] }
 iana-time-zone = "0.1"
+sha2 = "0.10"
 
 [build-dependencies]
 tauri-build = { version = "2", features = [] }