fix(terminal): bound live xterm backpressure

[dignfei]

Follow-up to #331.

Summary

• Apply backpressure to live xterm writes by waiting for each parse callback before handing off the next bounded chunk.
• Include queued, loading, in-flight, and incoming bytes in the 128 KiB client cap; keep automatic shell recovery on the bounded 1 MiB tail.
• Drop redundant SSE terminal payloads while WebSocket owns terminal I/O, and make recovery single-flight and session-safe.
• Document the bounded ordinary-shell scrollback behavior.

Testing

• 14 focused Vitest files: 251 tests passed.
• TypeScript type check passed.
• Frontend syntax check passed for 34 files.
• Prettier and staged diff checks passed.