Stabilize cmux jump and session rendering

[graykode]

Summary

• Add a cmux jump fallback that uses the target session cmux environment and AppleScript when the cmux socket rejects external commands.
• Repaint the session table before rendering rows to avoid stale selection markers while moving with arrow keys.
• Make UI truncation respect terminal display width and add regression coverage for wide characters and jump footer behavior.

Test Plan

• rustfmt --edition 2021 --check src/jump/cmux.rs src/ui/footer.rs src/ui/mod.rs src/ui/sessions.rs
• cargo test
• cargo clippy -- -D warnings
• cargo build
• cargo run -- --once
• Live PTY check: selected a cmux session, pressed Enter, verified cmux selected the target tab and focused terminal IDs; moved selection with Down and verified clean repainting.