VoiceOver is never told an AI suggestion is on screen, yet Tab and Escape are taken while it is

## What happens

An AI suggestion is drawn in a floating panel and never announced, so a VoiceOver user does not know one is on screen, yet the accept key is armed while it is.

- `SuggestionView` (`Sources/Uttrflow/Suggestion/SuggestionView.swift:23-24`) gives the surface a combined element and an `accessibilityLabel` built by `SuggestionPresentation.accessibilityLabel` (`Sources/UttrflowUX/SuggestionPresentation.swift:141-149`), e.g. "Suggestion: … Tab to accept."
- Nothing ever reads that label out. There is no `NSAccessibility.post(…, .announcementRequested, …)` anywhere under `Sources/Uttrflow/Suggestion/` (a search for `NSAccessibility.post` in `Sources/` finds none at all).
- The panel is a non-activating `NSPanel` at `.statusBar` level with `ignoresMouseEvents = true` (`SuggestionPanelController.swift:155-172`). VoiceOver's cursor stays in the other application's field and cannot reach it.
- Meanwhile `SuggestionCoordinator.draw` arms the tap (`SuggestionCoordinator.swift:594-608`), and `KeyRouting.decision` takes Tab (or the application's accept key) and Escape (`Sources/UttrflowPredict/KeyRouting.swift:85-94`).

So for a VoiceOver user, Tab in a web form or a mail field sometimes moves focus and sometimes silently types text they never heard, and Escape sometimes does nothing. Nothing checks `NSWorkspace.shared.isVoiceOverEnabled` either.

## Why it matters

A key that changes meaning based on something invisible breaks keyboard and screen-reader use. A blind user may send a message containing a completion they never heard. This is the core accept flow, so P1.

## How to reproduce

Code reading (above). With VoiceOver on and AI suggestions on, type a remembered line in a text field. VoiceOver says nothing about the grey text. Press Tab: the completion is inserted instead of focus moving.

## Acceptance criteria

- When a suggestion first appears for a line (not on every redraw), its text and accept key are announced with `NSAccessibility.post(element:notification:.announcementRequested, userInfo:)` at a low priority, so typing echo is not interrupted. The label that already exists is the text.
- Or, as a product decision, AI suggestions arm no keys while VoiceOver is running unless the user opts in. Either way a VoiceOver user is never given an accept key they were not told about.
- A test in `Tests/UttrflowUXTests` covers the announcement text for a single completion, a replacement and a list.

Start with SuggestionView.swift, SuggestionPresentation.swift, SuggestionPanelController.swift, SuggestionCoordinator.draw, and KeyRouting.decision to trace the suggestion lifecycle and existing label. Then inspect Tests/UttrflowUXTests and determine how to cover a single completion, replacement, and list; done means VoiceOver users are informed before the accept key is active, without repeated announcements.

Written by the indexing model from the issue text.