Expand test coverage across server, agent, and web; fix bugs found during audit.

Adds hundreds of unit/integration/e2e tests, fixes WS bcrypt auth, config merge, fleet analytics, agent schedule/log tail, and documents stale PROBLEMS items. Updates PROBLEMS.md, README, and test scripts; ignores local spread-kits and coverage dirs.

server/internal/builder/platform.go

@@ -46,11 +46,17 @@ func platformsForRequest(req *BuildRequest) []BuildPlatform {
 	}
 	if target == "universal" {
 		if req.TargetArch != "" && req.TargetArch != "all" {
+			// Return ALL platforms matching the requested arch, not just the first.
+			// e.g. arm64 → [linux-arm64, darwin-arm64], not just linux-arm64.
+			var matched []BuildPlatform
 			for _, p := range defaultPlatforms {
 				if p.GOARCH == req.TargetArch {
-					return []BuildPlatform{p}
+					matched = append(matched, p)
 				}
 			}
+			if len(matched) > 0 {
+				return matched
+			}
 		}
 		return append([]BuildPlatform{}, defaultPlatforms...)
 	}