feat: banner when a Bluetooth mic pairs + fix silent BT mic level meter

[joeblau]

Summary

Two related pieces of Bluetooth-mic UX in the streaming flow.

1. Banner when a Bluetooth audio device pairs mid-session

• AudioInputProvider diffs the Bluetooth input set on each AVAudioSession route change and fires a new onBluetoothConnected callback only for devices that arrive — already-connected devices at launch seed the baseline, so no false banner.
• The provider is hoisted from SettingsView into ContentView so its route-change observer runs for the whole app lifetime (not just while the Settings sheet is open); both screens share one instance/observer.
• A transient glass banner ("<name> connected" + haptic) slides in on the main streaming screen, stacked under the LIVE pill, and auto-dismisses after 3s.

2. Fix: Bluetooth mic level meter read silent

Confirmed on device — the meter was correctly routed (route=[DJI Mic 3 TX-01ffca/BluetoothHFP], 16 kHz) yet flat. Two root causes:

• A tap alone doesn't pull a Bluetooth HFP input. The engine only renders its input when the graph drives an output, so an unconnected input node hands the tap silent buffers. Input is now routed through the main mixer with output volume 0 (no monitoring/feedback) to force a full I/O cycle.
• No AVAudioEngineConfigurationChange observer. If the HFP link settled asynchronously after setPreferredInput, the tap stayed bound to the stale (built-in) format and went silent. Tap install is extracted into a reusable installMeterTap() that a new observer re-runs on reconfiguration; the observer is torn down in stopLocalCapture.

Testing

• xcodebuild build -scheme Stream -destination 'generic/platform=iOS' → BUILD SUCCEEDED.
• StreamCore test suite unaffected (changes are app-target only).
• ⚠️ The mic-meter fix is not yet confirmed on-device — the muted-mixer pull is the standard remedy for silent AVAudioEngine HFP input, but the DJI Mic 3's over-Bluetooth behavior should be verified live. If the DJI won't stream over BT HFP at all, that's a pairing limitation (use the USB-C receiver), not an app bug.

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