fix: resolve display stuck after disconnect and radio task deadlock

- Set disconnected flag on DisplayCommand::Reset to block stale radio
  status updates from re-rendering dashboard over splash screen
- Send StopRx to radio when USB DTR drops so radio stops generating
  packets after host disconnects
- Use try_send for unsolicited responses (RxPacket, error recovery,
  StopRx Ok) to prevent radio task deadlock on full response channel
- Fall back to LED-only event loop when display init fails instead of
  returning early and orphaning the LED

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

src/host/usb.rs

@@ -8,6 +8,7 @@ use embassy_usb::Builder;
 use static_cell::StaticCell;
 
 use crate::channel::{CommandChannel, DisplayCommand, DisplayCommandChannel, ResponseChannel};
+use crate::protocol::Command;
 use super::framing::{self, CobsDecoder, MAX_FRAME};
 
 const MAX_PACKET: usize = 64;
@@ -126,8 +127,11 @@ async fn protocol_loop<'d, D: embassy_usb_driver::Driver<'d>>(
         }
 
         let connected = receiver.dtr();
-        if was_connected && !connected && has_display {
-            display_commands.send(DisplayCommand::Reset).await;
+        if was_connected && !connected {
+            let _ = commands.try_send(Command::StopRx);
+            if has_display {
+                display_commands.send(DisplayCommand::Reset).await;
+            }
         }
         if !was_connected && connected && has_display {
             display_commands.send(DisplayCommand::On).await;