Add an optional Coach launcher card to Today

The problem

Coach is currently buried in the Insights/More navigation. It is easy to miss, particularly on iPhone, and entering it requires leaving Today.

The idea

Add Coach as an optional, off-by-default card in Today customization across iOS, macOS, and Android.

Tapping the card would open a compact platform-native Coach launcher:

iOS: modal sheet
macOS: sheet
Android: bottom sheet

When Coach is configured, the launcher would offer the existing suggested questions and a compact composer. When it is not configured, it would explain the opt-in, bring-your-own-key model and route to the existing Coach setup screen.

Constraints:

Opening the card or sheet must not make a network request.
Existing provider, key storage, consent, system prompt, BLE, and offline-by-default behavior remain unchanged.
Use the existing shared DashboardCard preference contract so card selection stays consistent across platforms.
Keep the existing full Coach screen as the configuration and fallback surface.

Alternatives you've considered

Leave Coach in More/Insights. This keeps Today uncluttered but leaves the feature hard to discover.
Add Coach as a permanent Today section. An optional card is preferable because users who do not use a provider should not get another fixed dashboard element.
Redesign Coach immediately as a persistent ChatGPT-style workspace. That is a larger follow-up: it needs local conversation/thread storage, retention and deletion controls, backup policy, and a decision about when prior turns may be sent to a provider.

Additional context

The first contribution should stay limited to the launcher. Persistent conversation history, training plans, a thread list, provider changes, and changes to consent behavior are out of scope.

Verification should confirm that the card is absent by default, can be added/moved/removed through Today customization, opens the expected platform-native surface, routes unconfigured users to existing setup, and does not make a provider request until the user explicitly sends a question.