feat: optional whisper, fix 1.7b model loading & tray error state

[jafreck]

Summary

Make whisper-rs an optional dependency behind a whisper feature flag, fix Qwen3-ASR 1.7b model loading, distinguish the tray Error state from Recording, and fix flaky config tests.

Optional Whisper (whisper feature flag)

• whisper-rs is now optional = true in murmur-core, gated behind a new whisper feature
• Default features changed to [onnx] — Qwen3-ASR is the default, slim builds exclude whisper entirely
• metal/cuda/vulkan features imply whisper (they're whisper-rs hardware backends)
• murmur-whisper-worker binary uses required-features = ["whisper"]
• Source modules (transcriber.rs, whisper_engine.rs, prompt::rank_vocabulary) gated with #[cfg(feature = "whisper")]
• Factory returns clear error message when Whisper backend is used without the feature
• Wake word detection gracefully degrades (requires whisper feature for the tiny.en model)

Fix Qwen3-ASR 1.7b model loading

The 1.7b model's config.json uses a nested structure (decoder.hidden_size = 2048) but the parser only read the top-level hidden_size, defaulting to 1024. This caused Got invalid dimensions for input: input_embeds errors during inference. The 0.6b model worked only by coincidence (its decoder hidden_size happens to be 1024).

Fix tray Error vs Recording icon

Error state previously used the same red icon as Recording, making failed engine loads look like stuck recordings. Error now uses the idle icon, and the error message is shown in the tray status text.

Fix flaky tests

test_cmd_set_model_valid and test_cmd_set_model_prints_download_hint raced on the shared config file. Fixed by adding a CONFIG_MUTEX for test serialization and eliminating the double Config::load() TOCTOU in cmd_set_model.

Build configurations

Config	Command
Slim (Qwen3 only)	cargo build -p murmur
Full macOS	cargo build -p murmur --features metal
Explicit whisper	cargo build -p murmur --features whisper