Onboarding's last page draws letter, digit and F-key shortcuts as "Key 2", "Key 18", "Key 96"

## What happens

Onboarding's last page draws the dictation shortcut as keycaps with its own key-name table:

```swift
// Sources/UttrflowUX/OnboardingPresenter.swift:429-437
private static let names: [UInt16: String] = [
36: "Return", 48: "Tab", 49: "Space", 51: "Delete", 53: "Escape",
]
private static func name(for keyCode: UInt16) -> String {
names[keyCode] ?? "Key \(keyCode)"
}
```

Settings draws the same shortcut with `SettingsShortcut.keycaps(for:)`, whose table names every letter, digit, punctuation key, F-key, arrow and keypad key (`Sources/UttrflowUX/SettingsShortcut.swift:52-76`). `OnboardingKeys.of` already calls `SettingsShortcut` for held modifiers and for the modifier caps (`OnboardingPresenter.swift:423-427`), just not for the key.

So a shortcut such as ⌥D, ⌃⌘1 or ⌥F5 is shown on "You're all set" as ⌥ **Key 2**, ⌃ ⌘ **Key 18**, ⌥ **Key 96**, while Settings shows D, 1 and F5. The page reads settings at draw time (`OnboardingFlow.page`, `Sources/UttrflowUX/OnboardingFlow.swift:109-111`), so this shows whenever onboarding is reopened after the shortcut was changed (for example by the speech model repair path, `AppDelegate.repairSpeechModel`).

## Why it matters

The last onboarding page exists to teach the one key combination the user needs. "Key 2" teaches nothing, and the two windows disagree about the same shortcut.

## How to reproduce

`OnboardingKeys.of(HotkeyBinding(keyCode: 2, modifiers: [.option]))` returns `["⌥", "Key 2"]`; `SettingsShortcut.keycaps(for:)` on the same binding returns `["⌥", "D"]`.

## Acceptance criteria

- `OnboardingKeys.of` returns exactly `SettingsShortcut.keycaps(for:)` for every binding, and the private `names` table is deleted.
- A test in `Tests/UttrflowUXTests/OnboardingPresenterTests.swift` checks a letter, a digit and an F-key shortcut on the ready page against `SettingsShortcut.keycaps(for:)`.
- Existing onboarding tests for ⌥Space and for Fn still pass.

## Where to start

- `Sources/UttrflowUX/OnboardingPresenter.swift:418-437`.
- Run `make verify` (export `DEVELOPER_DIR=/Applications/Xcode.app/Contents/Developer` first) and read CONTRIBUTING.md.

**Size:** XS, under an hour.

Start in Sources/UttrflowUX/OnboardingPresenter.swift:418-437 and compare OnboardingKeys.of with SettingsShortcut.keycaps(for:). Add the requested coverage in Tests/UttrflowUXTests/OnboardingPresenterTests.swift for a letter, digit and F-key, while preserving the existing Space and Fn tests. Run make verify with DEVELOPER_DIR set, and done means both paths return matching keycaps without the private names table.

Written by the indexing model from the issue text.