feat: add IdfotoError enum with thiserror

crates/idfoto-core/src/error.rs

@@ -1 +1,41 @@
-// Error types will be filled in Task 2
+use thiserror::Error;
+
+#[derive(Debug, Error)]
+pub enum IdfotoError {
+    #[error("key derivation failed: {0}")]
+    Kdf(String),
+
+    #[error("encryption failed: {0}")]
+    Encrypt(String),
+
+    #[error("decryption failed: wrong key or corrupted data")]
+    Decrypt,
+
+    #[error("invalid vault format: {0}")]
+    Format(String),
+
+    #[error("entry not found: {0}")]
+    EntryNotFound(String),
+
+    #[error("imgsecret: {0}")]
+    ImgSecret(String),
+
+    #[error("image too small: need at least {min_width}x{min_height}, got {actual_width}x{actual_height}")]
+    ImageTooSmall {
+        min_width: u32,
+        min_height: u32,
+        actual_width: u32,
+        actual_height: u32,
+    },
+
+    #[error("extraction failed: no valid secret found in image")]
+    ExtractionFailed,
+
+    #[error("json error: {0}")]
+    Json(#[from] serde_json::Error),
+
+    #[error("device key error: {0}")]
+    DeviceKey(String),
+}
+
+pub type Result<T> = std::result::Result<T, IdfotoError>;

crates/idfoto-core/src/lib.rs

@@ -1 +1,2 @@
 pub mod error;
+pub use error::{IdfotoError, Result};