[SUB-FEATURE] Implement `BrowserStorage` WASM backend

[takasaki404]

[SUB-FEATURE] Implement BrowserStorage WASM backend

📋 Summary

Provide a concrete Storage implementation that runs under wasm32-unknown-unknown, using the browser’s IndexedDB (with a LocalStorage fallback) to persist key/value blobs.

💡 Proposal

1. Module & Struct

   • Create amico-core/src/browser_storage.rs

   • Define

     pub struct BrowserStorage {
         db_name: String,
         // optionally hold a handle to `web_sys::IdbDatabase`
     }

2. Initialization

   • Feature-gate on cfg(target_arch = "wasm32")

   • In constructor, open (or create) an IndexedDB database named e.g. "amico-storage".

   • Expose an async constructor:

     impl BrowserStorage {
         pub async fn new(db_name: &str) -> Result<Self, StorageError> { … }
     }

3. Namespace & Key Mapping

   • Treat each namespace as an object store in IndexedDB.
   • Use key as the primary key in that store.
   • Store values as Uint8Array or base64-encoded strings for LocalStorage.

4. Trait Implementation

   • All methods become async fn returning Future<Output = Result<…, StorageError>>.
   • get: open transaction, fetch key, return None if missing.
   • put: open readwrite transaction, put or add the blob.
   • delete: open readwrite, call delete.
   • list_keys: open readonly, use a cursor to collect all keys.

   #[async_trait::async_trait]
   impl Storage for BrowserStorage {
       async fn get(&self, namespace: &str, key: &str) -> Result<Option<Vec<u8>>, StorageError> { … }
       async fn put(&mut self, namespace: &str, key: &str, value: Vec<u8>) -> Result<(), StorageError> { … }
       async fn delete(&mut self, namespace: &str, key: &str) -> Result<(), StorageError> { … }
       async fn list_keys(&self, namespace: &str) -> Result<Vec<String>, StorageError> { … }
   }

5. LocalStorage Fallback

   • If IndexedDB isn’t available (e.g. insecure contexts), fallback to window.localStorage.
   • Serialize the entire namespace map as a JSON object under a single LS key (e.g. "amico:<namespace>").

6. Testing

   • Write wasm-bindgen integration tests (e.g. with wasm-pack test --headless).
   • Cover all operations in a browser-like environment (headless Chrome or wasm-bindgen test runner).
   • Fallback LS behavior should be tested in a simulated environment where IDB isn’t present.

✅ Acceptance Criteria

• browser_storage.rs module with BrowserStorage struct and async new() constructor.
• Implements Storage trait (async) with correct namespace ↔ object store mapping.
• IndexedDB operations using web_sys APIs, with appropriate error conversions to StorageError.
• LocalStorage fallback path that preserves semantics.
• Integration tests exercising all four methods under both IDB and LS modes.
• Docs/comments explaining feature flag, async behavior, and fallbacks.