fix(tui): slash command polish and remove provider response timeouts

[wangtsiao]

Summary

• TUI: preserve the composer status footer when opening the slash command popup.
• TUI: remove the slash command selection marker and its leftover padding.
• Provider: remove per-provider response timeouts; rely on the existing recovery-hint flow instead of duplicating timeout handling.

Test Plan

• cargo test for TUI and provider changes.
• Desktop provider settings tests updated for the timeout removal.