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/web/src/pages/AgentsPage.tsx

@@ -269,6 +269,8 @@ export default function AgentsPage() {
               filters={filters}
               onChange={setFilters}
               selectedCount={selectedIds.size}
+              filteredCount={filteredAgents.length}
+              onSelectAllFiltered={() => setSelectedIds(new Set(filteredAgents.map((a) => a.id)))}
               onBulkAction={handleBulkAction}
               bulkBusy={bulkBusy}
             />