fix: reconnect to dispatcher DO and flush messages

[rpurdel]

Fixes a bug where on dispatcher DO disconnect it will silently drop messages.

1. Added message queue for transcriptions when dispatcher is disconnected (instead of dropping
   them silently)
2. Added reconnection logic with exponential backoff (1s → 2s → 4s → ... max 30s):

• connectToDispatcher() function to connect/reconnect to DO
• scheduleReconnect() function with exponential backoff
• flushMessageQueue() to send queued messages after reconnection

3. Added close/error handlers for dispatcher WebSocket to detect mid-session disconnects and
   trigger reconnection
4. Added flushBeforeClose() - when client disconnects, attempts to reconnect and flush any
   remaining queued messages using ctx.waitUntil()
5. Fixed reconnection trigger condition - changed from !dispatcherWs to !dispatcherWs ||
   dispatcherWs.readyState !== WebSocket.READY_STATE_OPEN so reconnection triggers even when
   WebSocket object exists but is CLOSED