feat: add streaming response support

[Isaccseven]

Summary

• Add token-by-token streaming display when stream="true" prop is set
• Consume Langflow's existing SSE streaming API (POST /api/v1/run/{flow_id}?stream=true)
• Bot messages render progressively instead of appearing all at once after completion
• Backward-compatible: defaults to false, existing behavior unchanged when omitted

Changes

• New: src/controllers/streamMessage.ts — streaming controller using fetch + ReadableStream with SSE parsing, AbortSignal support, and stream completion safety fallback
• Modified: src/index.tsx — register stream: "boolean" web component prop
• Modified: src/chatWidget/index.tsx — pass stream prop through to ChatWindow
• Modified: src/chatWidget/chatWindow/index.tsx — streaming branch in handleClick() with requestAnimationFrame batched token updates, AbortController cleanup on unmount, placeholder hidden during streaming

Usage

<langflow-chat
  host_url="https://my-langflow.com"
  flow_id="abc-123"
  stream="true"
/>

Test plan

• Verify stream="true" causes bot messages to render token-by-token
• Verify omitting stream prop preserves existing non-streaming behavior
• Verify input is disabled during streaming
• Verify mid-stream errors preserve partial text with error styling
• Verify session ID continuity across streamed messages
• Verify closing widget during stream doesn't leak connections

Closes #44

[Isaccseven]

Ready for review