Implement crypto adapter pattern for platform independence

Adds CryptoAdapter interface with WebCryptoAdapter (browser) and
NodeCryptoAdapter (Node.js 19+) implementations.

Changes:
- Created crypto-adapter.ts interface
- Created web-crypto-adapter.ts for browser environments
- Created node-crypto-adapter.ts for Node.js environments
- Updated RondevuAPI to accept optional CryptoAdapter
- Updated Rondevu class to pass crypto adapter through
- Exported adapters and types in index.ts
- Updated README with platform support documentation
- Bumped version to 0.15.0

This allows the client library to work in both browser and Node.js
environments by providing platform-specific crypto implementations.

Example usage in Node.js:
  import { Rondevu, NodeCryptoAdapter } from '@xtr-dev/rondevu-client'

  const rondevu = new Rondevu({
    apiUrl: 'https://api.ronde.vu',
    cryptoAdapter: new NodeCryptoAdapter()
  })

🤖 Generated with Claude Code
https://claude.com/claude-code

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/index.ts

@@ -7,6 +7,10 @@ export { Rondevu } from './rondevu.js'
 export { RondevuAPI } from './api.js'
 export { RondevuSignaler } from './rondevu-signaler.js'
 
+// Export crypto adapters
+export { WebCryptoAdapter } from './web-crypto-adapter.js'
+export { NodeCryptoAdapter } from './node-crypto-adapter.js'
+
 // Export types
 export type {
     Signaler,
@@ -26,3 +30,5 @@ export type { RondevuOptions, PublishServiceOptions } from './rondevu.js'
 
 export type { PollingConfig } from './rondevu-signaler.js'
 
+export type { CryptoAdapter } from './crypto-adapter.js'
+