fix(ui): improve chat timestamps and tui paste

[szdtzpj]

Summary

• restore Ctrl+V paste in tuiapp_v2.py while keeping bracketed paste newline normalization, long-paste placeholders, and duplicate-paste protection
• show visible timestamps for Streamlit chat messages, including live assistant replies and saved command responses
• add the stapp.py Home/End chat input shim from fix/修复 macOS 中 stapp.py ，streamlit 界面chat输入框不能用 Home / End 键跳转光标 #249
• add focused Textual regression tests for paste behavior

Issues

• Addresses tuiapp_v2.py: Copy and paste do not work in the TUI #351
• Addresses feat: add timestamps to each chat turn so users can tell whether GA is stuck #349 for the Streamlit chat UI
• Addresses fix/修复 macOS 中 stapp.py ，streamlit 界面chat输入框不能用 Home / End 键跳转光标 #249

Validation

• python -m unittest discover -s tests -p test*.py
• python -m py_compile frontends\tuiapp_v2.py frontends\stapp.py tests\test_tuiapp_v2_input.py
• python frontends\tuiapp_v2.py --help
• git diff --check

[szdtzpj]

Rebased this PR on the latest main (09cd857, which includes #352) and resolved the frontends/tuiapp_v2.py conflict by keeping the upstream unified bracketed-paste insertion path while preserving this PR's Ctrl+V support and duplicate-paste guard.

Re-ran validation locally after the rebase:

• python -m unittest discover -s tests -p test*.py
• python -m py_compile frontends\tuiapp_v2.py frontends\stapp.py tests\test_tuiapp_v2_input.py
• python frontends\tuiapp_v2.py --help
• git diff --check

[szdtzpj]

Superseded by #354.