feat: add settings command to lean TUI

[rumpl]

Summary

• Add an inline /settings panel with keyboard navigation, save/cancel, and persistence that preserves unrelated preferences.
• Include steering/queueing, default auto-approval, snapshots, cache-stable prompts, cache-miss warnings, and lean UI defaults.
• Add completion sounds and duration threshold, image rendering, startup banner, and split-diff controls; omit themes, sidebar layout, and tab settings.
• Wire completion/failure sounds into the lean event loop, without chiming on cancellations or nested stream completion.
• Document the panel and add coverage for persistence, cancellation, confirmation, sound playback, and rendering settings.

Validation

• task build — passed
• task lint — passed
• go test ./pkg/leantui/... — passed
• task test — blocked by unrelated pkg/acp failure: TestUsageBackgroundRuntimeRecordsWithoutClientIO expects cold-resume cost 0.00075 but receives 0. This also reproduces in isolation; ACP does not depend on the lean TUI.