fix(shared): drop .js extensions from relative imports for Metro

packages/shared/src/index.ts and all sub-modules used .js extensions on
relative imports (e.g. './admin/index.js') pointing at .ts source files.
TypeScript with moduleResolution: "Bundler" doesn't need them, and
Metro's eager exporter (used for preview / production builds) reads
them literally and fails — only the dev-server Metro fell back to .ts.

Workspace typecheck remains 8/8 green; Vite and TS Bundler resolution
already accept both styles, so desktop is unaffected.

packages/shared/src/crypto/box.ts

@@ -1,4 +1,4 @@
-import { getCryptoBackend } from './backend.js';
+import { getCryptoBackend } from './backend';
 
 // Authenticated encryption using XSalsa20-Poly1305 + X25519 (Curve25519).
 // Delegates to the active CryptoBackend (libsodium on desktop, libsodium-rn

packages/shared/src/crypto/index.ts

@@ -1,4 +1,4 @@
-export * from './backend.js';
-export * from './box.js';
-export * from './keys.js';
-export * from './sessionKeys.js';
+export * from './backend';
+export * from './box';
+export * from './keys';
+export * from './sessionKeys';

packages/shared/src/crypto/keys.ts

@@ -1,4 +1,4 @@
-import { getCryptoBackend } from './backend.js';
+import { getCryptoBackend } from './backend';
 
 export interface X25519KeyPair {
   publicKey: Uint8Array; // 32 bytes

packages/shared/src/crypto/sessionKeys.ts

@@ -1,7 +1,7 @@
-import { getCryptoBackend } from './backend.js';
-import { decryptFrom, encryptFor, type EncryptedEnvelope } from './box.js';
+import { getCryptoBackend } from './backend';
+import { decryptFrom, encryptFor, type EncryptedEnvelope } from './box';
 
-// Sender-Key (Signal-style) helpers — one symmetric XSalsa20-Poly1305 key per
+// Sender-Key (Signal-style) helpers — one symmetric XSalsa20-Poly1305 key per
 // conversation, wrapped with `crypto_box` for each recipient device's pubkey.
 //
 // Flow: