fix(devices): surface stalled and failed remote desktop connections

[posthog]

Problem

• A remote desktop connection failure was a silent dead end: the user got a black rectangle with no error, no retry, and no way out except reloading the page. This is a core, high-traffic flow.
• The canvas stayed visibility: hidden until the first JPEG tile drew, and the spinner rendered only while !firstFrameReceived && state >= 1. When the relay socket dropped, MeshTunnel reports state 0, so both conditions went false at once and the container showed its plain bg-black.
• No first-frame timeout: once the handshake reached state 3, nothing re-requested a key frame or reported a failure if the agent never streamed — an eternal "Waiting for desktop stream…".
• No visibility: nothing in src/lib/meshcentral/ captured an exception, so this whole class of breakage was invisible in error tracking.

Changes

Failure	Before	After
Relay socket drops before any frame (state 0)	Blank black canvas	Failed state with reason + Retry button
Agent connects but never streams (state 3)	Endless spinner	One re-send of the compression/start message, then "The desktop stream did not start."
Connect setup throws	Toast only	Failed state + Retry + $exception

• Retry clears the failure and re-runs the connect effect (connectNonce); its cleanup resets initializingRef and stops the dead tunnel, then a fresh tunnel reconnects.
• The state-0 handler ignores the teardown-emitted state 0 (cancelled guard) so a retry does not re-raise the failure it just cleared. Once a frame has drawn, the canvas keeps its last image and the existing reconnect toasts still apply.
• Each failure is reported through a new captureException helper (routes to the GTM-loaded window.posthog) with tunnelState and nodeId attached.

Why

The failure path ended in an unrecoverable dead end on a core flow, and the failure rate could not even be measured because the surface emitted no exception or success events.

Test plan

• npm run type-check passes.
• biome check clean on the changed files.
• Live remote-desktop reproduction (relay drop / silent agent) — needs a MeshCentral relay + agent, not available in the sandbox. Verified by tracing each state transition through the new rendering branches.

Notes

• Related but not overlapping: fix(): one contract for loading, offline, error and emptiness #213 reworks loading/offline/error/empty states across list and detail pages, but does not touch remote-desktop or src/lib/meshcentral/. This change is additive and specific to the remote desktop viewer.

---

Created with PostHog Desktop from this inbox report.