feat: entry update links existing secrets (link_secret_names)

- secrets-core: update flow validates and applies secret links
- secrets-mcp: MCP tool params and UI for managing links on edit
- Align errors and templates; minor crypto/.gitignore tweaks

Made-with: Cursor

crates/secrets-mcp/src/error.rs

@@ -28,6 +28,10 @@ pub fn app_error_to_mcp(err: &AppError) -> rmcp::ErrorData {
             "The entry was modified by another request. Please refresh and try again.",
             None,
         ),
+        AppError::DecryptionFailed => rmcp::ErrorData::invalid_request(
+            "Decryption failed — the encryption key may be incorrect or does not match the data.",
+            None,
+        ),
         AppError::Internal(_) => rmcp::ErrorData::internal_error(
             "Request failed due to a server error. Check service logs if you need details.",
             None,