Fix log stream dying silently across WiFi mode switches

[eliasbakken]

Summary

• `streamLog()` never checked for client disconnection - the handler (and its tail follower) ran forever even after the client was gone, leaking one goroutine per dropped connection
• Reproduced Log stream stops working after connecting to an external AP #95 directly: when the board's WiFi interface disappears mid-stream (AP/station switch), the connection goes completely silent on both ends - no RST, no read error. Neither side's error handling fires, so the client waits forever
• `streamLog()` now watches the request context and returns/cleans up on disconnect, and sends a 15s heartbeat event
• The client watches for the heartbeat with a 30s watchdog and reconnects itself if none arrives, instead of relying on `onerror` which never fires in this scenario

Closes #95

Test plan

• Live-tested on real hardware: opened a raw stream connection, switched the board through an AP/station round-trip, confirmed via `ss` that the server-side connection cleaned up with no lingering CLOSE_WAIT sockets and no leaked goroutines
• Confirmed the heartbeat event is actually emitted every 15s
• Confirmed a fresh connection works cleanly immediately after the round-trip
• User confirmed the log viewer keeps working across a live WiFi mode switch in the browser
• go test ./... and bats test/bats pass

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