fix: resolve all 41 pre-existing dashboard test failures

- AgentsView: add missing fetchModels/fetchAgentStats mocks, update tests
  for new multi-step NewAgentDialog and ActiveAgentsPanel dual rendering
- routes-diff: rewrite tests to match new resolveDiffBase() two-dot syntax
- routes.test: fix spec/revise test — done→triage is valid per VALID_TRANSITIONS
- activity-feed CSS: replace hardcoded rgba(88,166,255) with color-mix token,
  fix var(--error) → var(--color-error)
- server/webhook/websocket tests: add getDatabase() mock for AiSessionStore

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/dashboard/app/styles.css

@@ -2068,7 +2068,7 @@ body {
   gap: var(--space-xs);
   font-size: 11px;
   color: var(--todo);
-  background: rgba(88, 166, 255, 0.1);
+  background: color-mix(in srgb, var(--todo) 10%, transparent);
   padding: 2px 8px;
   border-radius: var(--radius-sm);
 }
@@ -2173,7 +2173,7 @@ body {
   flex-direction: column;
   align-items: center;
   gap: var(--space-md);
-  color: var(--error);
+  color: var(--color-error);
   text-align: center;
 }