docs: fix credential return type, add missing test dir, update plan date

- docs/API.md: load_credentials returns Zeroizing<String> (not String)
- docs/DEVELOPMENT.md: add src/test/ directory to project structure
- PLAN.md: update Last Updated to 2026-04-23, bump version to 4.4

https://claude.ai/code/session_01By1aj94LMM7muDV7AT4egk

PLAN.md

@@ -1058,6 +1058,6 @@ This project is free and open-source software licensed under GPL v3.
 
 ---
 
-**Last Updated**: 2026-04-17
+**Last Updated**: 2026-04-23
-**Document Version**: 4.3
+**Document Version**: 4.4
 **Status**: Ready to Implement - Milestone-Driven Plan

docs/API.md

@@ -353,12 +353,14 @@ Credentials are stored in the platform keychain (Windows Credential Manager, mac
 
 ```rust
 use onyx_core::webdav::{store_credentials, load_credentials, delete_credentials};
+use zeroize::Zeroizing;
 
 // Store credentials
 store_credentials("nextcloud.example.com", "username", "password")?;
 
-// Load credentials (returns Zeroizing<String> wrappers that wipe memory on drop)
+// Load credentials — returns Zeroizing<String> wrappers that wipe memory on drop
-let (username, password) = load_credentials("nextcloud.example.com")?;
+let (username, password): (Zeroizing<String>, Zeroizing<String>) =
+    load_credentials("nextcloud.example.com")?;
 
 // Delete credentials
 delete_credentials("nextcloud.example.com")?;

docs/DEVELOPMENT.md

@@ -72,6 +72,8 @@ onyx/
 │       │   ├── main.ts
 │       │   ├── app.css                 # Tailwind CSS 4 + theme
 │       │   ├── App.svelte
+│       │   ├── test/
+│       │   │   └── setup.ts
 │       │   └── lib/
 │       │       ├── screens/            # Full-page views
 │       │       ├── components/         # Reusable UI components