feat: Add setting to disable radio error toasts (#80)

[cwill747]

Closes #80.

CRC and other radio warnings were shown as toasts at the bottom of the window, covering the message compose box while typing.

Changes

• New show_radio_error_toasts setting, default off, with a toggle in Settings → Logging. Existing DBs pick up the default since settings persistence is generic over dataclass fields (no migration needed).
• Radio errors still reach the logs and the header status badge, so nothing is silently dropped.
• The header badge is now transient. It previously latched on "Radio Error" until the next connect/disconnect, so one CRC warning hid the Connected/Offline state for the rest of the session — with toasts off by default that would have been the only feedback channel. It now reverts to live connection state after 5s, and repeated errors reset the timer rather than stacking timeout sources.
• The badge's accessible label is updated alongside its text (previously the error path skipped it, so a screen reader still announced "Connection status: Connected").

Testing

uv run pytest — 80 passed. ruff check / ruff format --check clean. New store test covers the default, a persisted False surviving the string round-trip, and True.

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