feat(dashboard): check GitHub for a newer release or newer commits

A worker thread asks GitHub's API about update_check_repo: the latest release
and, for development and CI builds, how far the build's commit is behind its
branch (or main). Unauthenticated, with ETags, every update_check_interval_hours
(default 6); offline or rate limited, the last answer stays next to the error
and a used-up limit waits for its reset. The main loop logs results, sends
them to master (UPDATE_STATUS, again when the link comes back) and pushes
update_check to open pages. GET /api/update_check, and
POST /api/update_check/check (settings permission, once a minute). Nothing is
downloaded or installed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

dDashboardServer/notify/UpdateChecker.h

@@ -0,0 +1,29 @@
+#pragma once
+
+#include "json_fwd.hpp"
+
+/**
+ * Checks GitHub for a newer release of the configured repository (update_check_repo) and, for development builds,
+ * how many commits the build's branch has that the build doesn't. Nothing is ever downloaded or installed.
+ *
+ * Unauthenticated and gentle: at most every update_check_interval_hours (and one "Check now" a minute), with ETags so
+ * unchanged answers don't count against GitHub's rate limit. Requests run on a worker thread; results come back in
+ * Update() on the main loop, which logs them, tells master (UPDATE_STATUS, for its log) and pushes update_check to
+ * open pages. Offline or rate limited, the last good answer stays and the error is shown next to it.
+ */
+namespace UpdateChecker {
+	void Initialize();
+	void Shutdown();
+
+	// Main loop: start a check when one is due, take in finished ones. masterConnected: the link to master is up (the
+	// last result is sent again each time it comes up, so a restarted master logs it too)
+	void Update(bool masterConnected);
+
+	// Check as soon as possible (no more than once a minute). Returns false when that was too soon or checks are off.
+	bool CheckNow(std::string& error);
+
+	// The running build and what the last check found, for the About page and /api/update_check
+	nlohmann::json Json();
+
+	void RegisterRoutes();
+}