feat(app): run gev on Navara behind ?engine=navara

The page's engine is ?engine=cesium|navara, else localStorage gev.engine,
else Cesium (src/app/engineChoice.js). Cesium mode is unchanged.

In Navara mode (src/app/navaraMode.js, loaded with import() only then,
and loading the Navara globe the same way) the scene builds the Navara
globe on the app container with no viewer; map stacks go through
engine.maps (Google 3D from the browser key, else Esri imagery); only the
shell's own data credits are registered. The layer catalog is empty and
the controls get empty layer stand-ins. The shell does not create its
viewer-bound parts (cockpit view, celestial ring, detection, tracked
readout, post-process stages); the tools install the render governor on
the engine, skip the director, annotations, drawing, imagery box, scope
mask, voice and embedded views, skip the first-run welcome, and show
"Experimental Navara engine: layers, cockpit, director and voice are
unavailable". window.__godsEyeView.viewer is null.

Navara's attribution card is kept clear of the location tray. A few
shell paths became null-safe for the missing viewer, layers or stages
without changing Cesium behaviour.

scripts/qa-engine-shell.mjs checks both modes headless: boot, viewer,
console errors, map drawn, search and fly, north/tilt/zoom/reset, orbit,
HUD, share link restore and map stacks.

src/ui/visualEffects.js

@@ -52,8 +52,10 @@ export class VisualEffects {
     this.previousBloom = null;
   }
 
+  // Post-process stages are Cesium's: without a viewer (the Navara engine)
+  // there are none, and the style, bloom and sharpen controls change nothing.
   initStyles() {
-    if (this.stopped || this.stageEntries.length) return;
+    if (!this.viewer || this.stopped || this.stageEntries.length) return;
     for (const [name, shader] of Object.entries(STYLES)) {
       const uniforms = { intensity: 0.0 };
       if (shader.fragmentShader.includes('uniform float time'))
@@ -73,7 +75,7 @@ export class VisualEffects {
   }
 
   initPostProcess(sharpenIntensity = this.sharpenIntensity) {
-    if (this.stopped || this.sharpenStage) return;
+    if (!this.viewer || this.stopped || this.sharpenStage) return;
     this.bloomStage = this.viewer.scene.postProcessStages.bloom;
     const names = [
       'glowOnly',
@@ -224,6 +226,7 @@ export class VisualEffects {
     if (this.destroyed) return;
     this.stop();
     this.destroyed = true;
+    if (!this.viewer) return;
     const stages = this.viewer.scene.postProcessStages;
     for (const [, stage] of this.stageEntries) stages.remove(stage);
     if (this.sharpenStage) stages.remove(this.sharpenStage);