fix(fsapp): rollover oversized Feishu task cards

[hanshou101]

Summary

• continue long-running Feishu task updates on a new interactive card when message.patch hits the platform element limit
• keep the existing single-card behavior unchanged until the limit is actually hit
• preserve global turn numbering across continuation cards

Why

Long-running tasks can keep executing in the backend while the visible Feishu working card freezes on an older turn. The direct cause is message.patch failing with platform card-size/card-element-limit errors such as 230099 / 11310 / element exceeds the limit.

This change keeps the original card as history and opens a continuation card for later updates, so users can still follow progress instead of seeing a stale Turn N card forever.

Scope

• only touches frontends/fsapp.py
• does not change agent loop / LLM / timeout semantics
• does not change normal short-task behavior

Validation

• python3 -m py_compile frontends/fsapp.py
• simulated single-card case: unchanged
• simulated overflow on a step update: rolls over to page 2
• simulated overflow on final output: rolls over to a new card and preserves the final text