agent: wayland — evdev hotkey backend, download resume, SHA256 integrity

Add kon-hotkey crate with evdev-based global hotkey capture that works on
Wayland (and X11). Patterns from whisper-overlay: per-device async listeners,
inotify hotplug with udev permission retry, watch channel for live config
updates. Frontend detects Wayland at startup and selects evdev or
tauri-plugin-global-shortcut automatically.

Model downloads now support HTTP Range resume for interrupted downloads and
optional SHA256 integrity verification (incremental, no second pass).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/transcription/Cargo.toml

@@ -16,3 +16,6 @@ tokio = { version = "1", features = ["rt", "sync"] }
 # Model downloads
 reqwest = { version = "0.12", features = ["stream"] }
 futures-util = "0.3"
+
+# Download integrity verification
+sha2 = "0.10"