chore: cleanup sprint — dead CSS, dedup helpers, handler refactor

Remove ~126 lines of orphaned CSS from tile slim-down and old detail
layout. Consolidate 4 duplicate duration formatters into shared
elapsed()/fmtElapsed() helpers. Break 160-line Result handler into
focused sub-functions. Implement real Hub.Shutdown() (was a no-op).
Standardize agent error responses to JSON. Replace panic() in router
init with error return. Extract magic numbers as named constants.

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

internal/httpserver/sse_e2e_test.go

@@ -72,7 +72,10 @@ func TestSSE_EndToEnd(t *testing.T) {
 		t.Fatalf("create host: %v", err)
 	}
 
-	router := NewRouter(Deps{UI: ui, Agent: agent})
+	router, err := NewRouter(Deps{UI: ui, Agent: agent})
+	if err != nil {
+		t.Fatalf("router: %v", err)
+	}
 	srv := httptest.NewServer(router)
 	t.Cleanup(srv.Close)
 
@@ -178,7 +181,10 @@ func TestSSE_SubStepEvent(t *testing.T) {
 		SpecDiffs: diffs, Runner: runner, EventHub: hub,
 	}
 
-	router := NewRouter(Deps{UI: ui, Agent: agent})
+	router, err := NewRouter(Deps{UI: ui, Agent: agent})
+	if err != nil {
+		t.Fatalf("router: %v", err)
+	}
 	srv := httptest.NewServer(router)
 	t.Cleanup(srv.Close)