feat: API-call action for buttons and sliders

[PalmarHealer]

Summary

Adds a configurable API call action to the button/slider wizard, so a button press or slider movement can fire an HTTP request.

• Method GET/POST/PUT/PATCH/DELETE, endpoint URL, optional JSON payload, optional bearer token.
• Sliders: the {value} placeholder in the URL and payload is replaced with the live slider percent (0–100).
• Per-action update interval (50–1000 ms, default 500) throttles slider-driven requests and always flushes the final value once the slider stops. Validated in the wizard (Next is gated + a hint when out of range). Buttons fire once per press, so they don't use the interval.

Implementation

• New ApiCall model (method, url, payload, bearer, throttle_ms) on ButtonAction and AudioStream; ActionKind::ApiCall.
• Cmd::ApiCall executed on a detached thread in the actuator so a slow endpoint never stalls audio. Uses ureq (blocking, rustls TLS).
• Engine throttles slider API calls per slider using each action's interval (smallest if several), with a trailing flush.
• Wizard UI: method dropdown, URL/payload/bearer fields, slider-only interval with range validation.

Compatibility

Existing presets load unchanged — new fields use serde defaults.

Tests

cargo test — 14 passed (4 new: {value} substitution for sliders vs. buttons, HttpMethod index round-trip, ApiCall label/stream).

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