feat(ui): separate dev info into a Diagnose view, declutter the scan workspace

Reworks the UI so the Scan tab shows only core actions and all developer /
diagnostic surfaces live in one dedicated view.

- New "Diagnose" nav view. ScanView stays mounted for scan|diagnose (the scan
  controller state persists across the switch) and renders either the clean
  workspace or the new DiagnosticsView by mode.
- DiagnosticsView consolidates runtime/rights, grid detection, learning + data
  staleness, fingerprint, auto-scan counters, the automation log, the dev-output
  toggle, the Demo-Daten action, and the raw crops/OCR/confidence dump. Reuses
  the existing diagnostics/details model hooks.
- Scan workspace decluttered: removed the Scanner Diagnose button and the Details
  button, dropped the rules/grid/mode dev fields from the result brief and capture
  meta, shortened the topbar headline, and moved Demo-Daten out of the topbar.
- Removed the now-dead ScanDiagnosticsModal / ScanDetailsModal components (their
  content moved into the view); metrics grid hidden on the Diagnose view.
- Added dev:web script + .claude/launch.json for browser preview.

Verified in the browser preview (both views render, no console errors); 120
tests + build green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/pages/app/AppPageLayout.tsx

@@ -53,9 +53,12 @@ export function AppPageLayout({ controller }: AppPageLayoutProps) {
             overlayIcon={<Eye size={16} />}
             demoIcon={<Play size={16} />}
           />
-          <AppMetrics metricCards={metricCards} />
-          {activeView === "scan" && (
+          {activeView !== "diagnose" && <AppMetrics metricCards={metricCards} />}
+          {(activeView === "scan" || activeView === "diagnose") && (
             <ScanView
+              mode={activeView === "diagnose" ? "diagnose" : "workspace"}
+              onDemoScan={handleDemoScan}
+              canDemoScan={!isScanning}
               isScanning={isScanning}
               snapshot={snapshot}
               captureSources={captureSources}