feat(webapp): add session persistence via URL hash routing + sessionStorage

- app.js: hash-based router (#runs / #new / #profiles / #report/{runId})
  - navigate() pushes history entries for back/forward support
  - _restoreSession() reads hash on load and popstate
  - sessionStorage fallback for same-tab refreshes
  - run-card highlights selected run (.run-card.selected)
- runner.js: use App.navigate() for report redirect; persist lastRunId to sessionStorage
- index.html: report nav button starts disabled (enabled on run select/restore)
- app.css: .run-card.selected with petrol border + ring

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

webapp/static/js/runner.js

@@ -10,9 +10,8 @@ const Runner = {
     document.getElementById("run-btn").addEventListener("click", () => Runner.trigger());
     document.getElementById("view-report-btn").addEventListener("click", () => {
       if (Runner.lastRunId) {
-        App.currentRunId = Runner.lastRunId;
         App.enableReportNav();
-        App.switchView("report");
+        App.navigate("report", Runner.lastRunId);
       }
     });
   },
@@ -164,6 +163,7 @@ const Runner = {
           runBtn.disabled = false;
           if (status.status === "completed" && status.run_id) {
             Runner.lastRunId = status.run_id;
+            sessionStorage.setItem("rag_run_id", status.run_id);
             reportBtn.hidden = false;
           }
         }