[chuks-qua]

What

Before, the model picker had no number shortcuts. Picking a model meant arrowing down or reaching for the mouse.

Now, while the picker is open, Ctrl+1..9 (Cmd+1..9 on macOS) pick the Nth selectable row of the active tab and close the picker. This works on every tab, Favourites included. Each numbered row shows its keycap. Disabled or gated rows get no number and do not use one up. Search renumbers the rows that remain.

New favourites now append instead of prepend. Starring a model no longer shifts every existing favourite down a row, so a favourite keeps its number. The order already saved on disk loads as it is, with no migration. At the 40-entry cap, the oldest favourite drops.

Picker gains an opt-in numberedShortcuts prop. Only ModelSelector turns it on, so the other pickers are unchanged.

Verified with tests and in the running web app:

• Picker tests cover numbering that skips disabled rows, Ctrl and Meta both picking, renumbering after search, and no effect without the prop.
• ModelSelector tests cover Ctrl+2 picking and closing. A second test runs the real global shortcut handler and shows that Ctrl+1 with the picker open picks a model and does not switch threads. That test first checks that Ctrl+1 outside the picker does reach thread.goTo1.
• New store tests cover appending, unstarring, a persisted order loaded from storage, and the 40 cap.
• Reverting the store change and the numberedShortcuts opt-in made 6 of these tests fail.
• In the live web app, the Codex tab showed Ctrl+1 to Ctrl+7. Pressing Ctrl+2 switched the model to GPT-6 Astra and closed the picker.
• Live keyboard check: arrow keys, Home, End, and Enter still move and pick. Clicking a tab and then pressing Ctrl+2 picks from that tab. Searching "luna" renumbers to Ctrl+1 and Ctrl+2. The Favourites tab numbers starred models and Ctrl+2 picks the second one.

Why

Users switch models many times a day. Number keys make that a single chord once the picker is open (#2226, part of #1813).

UI Changes

Before and after: the model picker, Codex tab, web app. The after shot shows a keycap on each selectable row.

Review Notes

• The chord is handled on the picker panel, not the search field, because clicking a tab moves focus onto the tab. The panel stops the event, so neither the browser's Ctrl+N tab switch nor the global thread.goTo* shortcut sees it. The global handler is unchanged.
• Past the 40 cap, the oldest favourite is dropped. Before this PR, the newest-first order dropped the last entry, which was also the oldest. Say if you would rather refuse a 41st star instead.
• The shortcut help dialog does not list this chord, just as it does not list Ctrl+D. The keycaps document it in place.
• Electron has no Ctrl+digit accelerators, so the desktop shell does not intercept the chord.

Closes #2226

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