feat(FN-5204): add test mode activation across settings, engine, and dashboard

- Add core settings schema/types support for testMode with model-resolution override handling
- Enforce engine session lane overrides in test mode with targeted helper coverage
- Add dashboard settings toggle plus persistent test-mode banner and related component tests
- Update settings documentation and parity/roundtrip tests for the new test mode behavior

packages/core/src/settings-schema.ts

@@ -17,6 +17,7 @@ export const DEFAULT_GLOBAL_SETTINGS = {
   dashboardFontScalePct: 100,
   defaultProvider: undefined,
   defaultModelId: undefined,
+  testMode: undefined,
   fallbackProvider: undefined,
   fallbackModelId: undefined,
   defaultThinkingLevel: undefined,
@@ -192,6 +193,7 @@ export const DEFAULT_PROJECT_SETTINGS = {
   groupOverlappingFiles: true,
   overlapIgnorePaths: [],
   autoMerge: true,
+  testMode: undefined,
   mergeStrategy: "direct",
   directMergeCommitStrategy: "auto",
   mergeIntegrationWorktree: "reuse-task-worktree",