fix(daemon): drain UI channel in headless mode to prevent panic

[DevenDucommun]

The headless display (PinePhone) was dropping the UI update receiver immediately without consuming it. This caused sends from diag.rs to fail with SendError and panic. Fix: spawn a drain task in headless mode that consumes messages until shutdown, consistent with all other display implementations.

Affected location:

• daemon/src/display/headless.rs

Fixes #657

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[DevenDucommun]

Fixing panics when notification/UI channels close during shutdown. When the receiver drops (e.g. during graceful shutdown), the daemon panics with an .expect() backtrace instead of handling it cleanly. This replaces those with warn!() logging so it exits gracefully.

This contribution was developed with AI assistance. I have reviewed, understood, and tested the change.

[untitaker]

i think the fix is to not return in the pinephone UI code. this fix would just spam warnings when running on pinephone.

[DevenDucommun]

You're right, updated the fix. The headless display was dropping the receiver immediately, so now it spawns a task to drain the channel until shutdown. Single file change, no more warn!() spam.

[untitaker]

I assume you didn't test it but this feels simple enough to merge as-is.

one note though: please don't let AI write comments for you