a11y: example-dashboard radiogroup lacks roving tabindex + arrow-key navigation

Where

src/ui/file-menu.ts — openExampleDashboardDialog (the "Import example dashboard…" picker added in #506).

What

The example-dashboard picker uses role="radio" rows inside a role="radiogroup" — the first use of that ARIA pattern in this codebase. Per the WAI-ARIA Authoring Practices, a radiogroup is expected to support:

• roving tabindex — only the checked (or first) radio is a Tab stop; the rest are tabindex="-1"
• arrow-key navigation (Up/Down or Left/Right) that both moves focus AND changes the selection between radios

Currently every row is a plain <button>, each independently Tab-stoppable, with no arrow-key handling. Functionally it still works today — Tab reaches each row in DOM order, Enter/Space activates it — so this isn't broken, just a deviation from the interaction model a screen-reader user would expect from the radio/radiogroup roles it advertises.

Why deferred

Out of scope for #506, which only requires "a single-select list… Import disabled until selected" — not full ARIA-pattern conformance. Flagged by an independent review pass during #506's implementation (PR TBD) as non-blocking.

Suggested fix

Add roving tabindex (tabindex="0" on the checked/first row, "-1" elsewhere) and an ArrowUp/ArrowDown (or Left/Right) handler on the radiogroup container that moves focus + selection together, matching the standard APG radiogroup example.

Start in src/ui/file-menu.ts at openExampleDashboardDialog and compare the radiogroup behavior with the WAI-ARIA Authoring Practices radiogroup example. Make only the checked or first row tabbable, keep other rows at tabindex="-1", and ensure arrow-key navigation moves both focus and selection between rows.