Open the project chooser without the keyboard on phones

[Tryanks]

Behaviour

On a phone, the Threads page "+" button opens the command palette as a project chooser when more than one project exists. The palette always focused its search input on open, which raised the software keyboard over a list that is meant to be tapped. The chooser now opens with the search input unfocused on mobile builds; the palette overlay takes focus instead so Back/Escape still close it. The search pill keeps raising the keyboard, and a narrow desktop window keeps typing into the chooser.

Changes

• WindowState records whether the palette should focus its query on open (open_palette vs the new open_palette_without_keyboard). This is the one owner of "how the palette opens"; the shell's frame-of-open focus step reads it.
• CommandPalette::focus became open(focus_query, ..): the reset (empty query, first row) is unchanged; only where focus goes differs.
• AppShell::start_thread picks the keyboard-less variant on mobile builds.

Tests

project_choice_reuses_drafts_and_focuses_only_on_desktop now asserts, after opening the chooser, that the query input is focused exactly when the build is not mobile, and that the palette owns focus in both cases. The added assertion fails without the fix (project chooser raises the keyboard only off-phone).

Checks run

• cargo fmt --all --check
• cargo clippy -p tcode-ui --all-targets --locked -- -D warnings
• cargo nextest run -p tcode-ui for the palette and shell focus tests

Not run locally: full workspace tests, iOS/Android/Web checks (CI covers them; the change is confined to crates/ui). Not exercised on a device.