Add keyboard shortcuts for thread navigation - #429

On macOS, Command+1 through Command+9 open a thread by its position in the displayed thread list. On Windows and Linux, the same navigation uses Ctrl+1 through Ctrl+9.

On all three platforms, Ctrl+Tab opens the next thread and Ctrl+Shift+Tab opens the previous thread, wrapping at either end of the list. While the model picker is open, its number shortcuts continue to select models. Pressing a number greater than the number of listed threads does nothing.

The sidebar shares row selection between rendering and navigation, keeping filters, collapsed groups, child threads and settled sections consistent in wide and compact layouts. Offscreen rows count; folded-away rows do not. GPUI tests cover rendered ordering, disclosures, wrapping, focus, Settings navigation and model-picker precedence using each platform's number modifier.

Validation on Linux:

• Latest change: cargo fmt --all --check and cargo test -p tcode-ui --locked thread_shortcuts_follow_list_order_and_leave_model_picker_numbers_alone passed.
• Before the modifier change: cargo build --workspace --locked and cargo test -p tcode-ui --locked passed (411 tests).
• cargo test --workspace --locked stopped at the unchanged late_terminal_attach_reproduces_the_host_grid test (terminal title zsh versus sh).
• cargo clippy --workspace --all-targets --locked -- -D warnings found five warnings in unchanged runtime command validation, UI store and terminal drawer code.
• cargo machete unavailable locally. macOS execution and cross-platform checks remain for CI.