Green: unseal shared collection keys with the account keypair

decryptCollection now takes the full key material {masterKey,
publicKey, secretKey} and dispatches on keyDecryptionNonce: present
means an owned collection (secretbox under the master key), absent
means a shared collection (sealed box to our public key). Client
already held the keypair for unsealing the auth token, so it just
passes it through.

src/model/types.ts

@@ -50,13 +50,25 @@ export interface EnteFile {
     updationTime: Microseconds;
 }
 
+// The key material a logged-in client holds, everything needed to decrypt
+// any collection: the master key (secretbox for owned collection keys) and
+// the X25519 keypair (sealed box for collection keys shared with us).
+export interface KeyMaterial {
+    masterKey: Uint8Array;
+    publicKey: Uint8Array;
+    secretKey: Uint8Array;
+}
+
 // Raw shapes as they arrive from the Ente API, before decryption.
 
 export interface RawCollection {
     id: number;
     owner: { id: number };
     encryptedKey: string;
-    keyDecryptionNonce: string;
+    // Absent for collections shared with us: their encryptedKey is a
+    // sealed box to our public key, which embeds an ephemeral public key
+    // instead of using a nonce.
+    keyDecryptionNonce?: string;
     encryptedName?: string;
     nameDecryptionNonce?: string;
     type: string;