fix(desktop): ⌘1–9 switch side-panel tabs inside the side panel

[kitlangton]

Kit asked for the browser-style shortcuts to follow focus into the side panel: ⌘T (#57156) and ⌘⇧T (#57269) did; ⌘1–9 didn't.

Now, while focus is in the side panel, ⌘1–8 select side-panel tab N and ⌘9 the last one, as in Chrome. Outside the panel, ⌘1–9 switch session tabs as before.

• sideTabNumbers() in closed-tabs.ts builds nine hidden, untitled commands (file.tab.1–file.tab.9) scoped with the same createSideFocus() predicate as ⌘⇧T, so they stay out of Settings › Shortcuts and win over the session tab.N binds only inside the panel. Keys match by typed character, so Dvorak is unaffected.
• They reuse the ⌃Tab/⌃⇧Tab side-tab cycling's selection and focus handling (selectSideTab), and are only registered while the side panel is open.
• A focused web page already forwards ⌘1–9 to the pane (fix(app): keep app shortcuts working inside the browser pane #56697), and the pane dispatches them on its root, so they land in the side panel even after clicking outside.

Stacked on #57269 (now merged); rebased onto beta.

Test: a component story — ⌘1 forwarded from a focused page after clicking outside selects side tab 1, ⌘2/⌘9 inside the panel select tab 2/the last, and ⌘2 outside still switches the session tab.