Make the Notch island the default dictation window

[r3dbars]

Justin's call for 1.1.67: the Notch island becomes the default dictation window.

What changes

DictationOverlayPresentationPreferences.defaultMode goes from .nearText to .notchIsland. The default is only read when nothing is saved, so:

• Never picked a window: gets the island on update. That's most people.
• Picked one (Near text box, Mini cursor or the island): keeps it.
• Near text box and Mini cursor stay in Settings → General, for anyone who wants the old look back.
• An unreadable saved value now falls back to the island as well.

Every reader goes through mode(): Settings, the island controller, the capture pill and the floating overlay. No screen keeps its own hard-coded default.

How I checked it

• New tests, written first and red before the change: no saved value gives the island, and reading the default saves nothing; every explicit pick survives; an unreadable value falls back to the island.
• bash check.sh: build, run-tests.sh (19654/19654), test-shape guard, concurrency census, source pins.
• Justin has used the island as his daily window for two days, on a 14" MacBook Pro and an M1.

Release note: people who never changed the dictation window will see the island after updating.

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