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/hotkey/src/stub.rs

@@ -0,0 +1,32 @@
+//! No-op stub for non-Linux platforms.
+//!
+//! On macOS and Windows, Tauri's global-shortcut plugin handles hotkeys
+//! natively. This stub exists so the crate compiles on all platforms.
+
+use tokio::sync::mpsc;
+
+use crate::HotkeyCombo;
+
+/// Events emitted by the hotkey listener.
+#[derive(Debug, Clone)]
+pub enum HotkeyEvent {
+    Pressed,
+    Released,
+}
+
+/// Stub listener that does nothing on non-Linux platforms.
+pub struct EvdevHotkeyListener;
+
+impl EvdevHotkeyListener {
+    pub fn start(
+        _combo: HotkeyCombo,
+        _event_tx: mpsc::Sender<HotkeyEvent>,
+    ) -> Self {
+        log::info!("evdev hotkey listener is a no-op on this platform");
+        Self
+    }
+
+    pub fn set_hotkey(&self, _combo: HotkeyCombo) {}
+
+    pub async fn stop(&self) {}
+}