feat: add opt-in starred-contact DND bypass with settings toggle

[blue-archon]

What

Adds an opt-in "Override DND for Favorites" toggle (default off) in Notification Settings. When enabled, a message from a starred/favorite Android contact breaks through Do Not Disturb; non-favorites follow normal DND.

How

Replaces the global channel.setBypassDnd(true) on the New Messages channel (and the per-conversation settings channel), which bypassed DND for every sender, with per-notification logic: when the sender resolves to a starred contact, the notification Person is built with setUri(lookupUri) plus setImportant so Android's own DND rules grant the exception contact by contact.

A new ContactNotificationHelper resolves the lookup URI and starred status. nativeContactId is resolved in Dart from the sender's ContactV2 and passed with the toggle through the create-incoming-message-notification channel, so there is no extra native contacts query and no SharedPreferences dependency.

Stacked on #3062.

[blue-archon]

Rebased onto current development and simplified: settings are now natively readable and the sender Person is already built in CreateIncomingMessageNotification, so I dropped the SharedPreferences workarounds from the earlier version. About 10 files instead of 21.

[blue-archon]

Rebased onto current master. Two things worth flagging:

• Resolved the notification-panel conflict from the settings rewrite. The "Override DND for Favorites" toggle now sits inside the new Obx-wrapped SettingsSection.
• Unstacked this from fix: don't hide group messages when the participant list is stale #3062. They touch disjoint files, so this is now just the DND feature on its own and can be reviewed independently.

[blue-archon]

Rebased onto v2.1.0, applies cleanly. This is the fork's headline feature, and I'd like to land it upstream so the fork can shrink. It uses Android's native starred-contact DND exception instead of putting the whole channel on the override list. Happy to adjust the approach if you'd like changes.