feat(chat): Ctrl+Enter to start session, arrow-key model nav (T-51)

[jaesbit]

Why

T-51: the "New session" modal (SessionLaunchModal) had no keyboard shortcut to submit and no way to browse the model list without the mouse.

Scope

• Ctrl+Enter (also Cmd+Enter) anywhere in the modal starts the session, mirroring the primary button.
• Arrow-key navigation (↑↓) through the model list in the search input, with Enter to pick the highlighted entry. Mouse hover stays in sync with the keyboard highlight.

Surface area

apps/web/src/components/chat/SessionLaunchModal.tsx only. ModelPickerModal.tsx (switch-model-on-existing-session) is untouched — out of scope per the ticket.

Verification

• bun run --filter '@omp-deck/web' typecheck — pass
• bun run --filter '@omp-deck/web' build — pass
• No React Testing Library precedent exists anywhere in this repo (confirmed via search — zero .test.tsx files, no @testing-library/* dependency), so no component test was added; traced the logic manually instead.