Make routing work from a file-served Capacitor bundle (#24) (#27)

## Summary
- Closes #24
- Introduce `AppRouter` + `getAppRouter()` so Capacitor uses `HashRouter` (filesystem bundle, no nginx rewrite) while web keeps `BrowserRouter`
- Isolate platform checks in `nativePlatform` (no `Capacitor.isNativePlatform()` sprinkled through the route tree)
- Auth forced redirects (`axiosApi` → `/signin/`) use hash-safe `redirectToAppPath` on native
- Leave CRA `homepage` / absolute `PUBLIC_URL` asset paths unchanged so `/var/www/{env}.chat.aimloperations/html` deploy stays intact; HashRouter keeps document URL at the bundle root so assets resolve

## Test plan
- [x] Unit tests: `AppRouter` + `nativePlatform` (`npm run test:ci`)
- [ ] Capacitor build: open nested route (`/#/account/`), force WebView reload — no blank/404
- [ ] Browser back/forward + Android hardware back with HashRouter
- [ ] Web deploy smoke: deep link reload at `/account/` still works via nginx rewrite
- [ ] Confirm login redirects after 401 still land on sign-in (web path + native hash)Reviewed-on: #27

llm-fe/src/react-app-env.d.ts

@@ -1 +1,19 @@
 /// <reference types="react-scripts" />
+
+interface CapacitorBridge {
+  isNativePlatform?: () => boolean;
+  isNative?: boolean;
+  Plugins?: {
+    App?: { addListener?: (...args: unknown[]) => unknown };
+    Network?: { addListener?: (...args: unknown[]) => unknown };
+    Preferences?: {
+      get?: (options: { key: string }) => Promise<{ value?: string | null }>;
+      set?: (options: { key: string; value: string }) => Promise<void>;
+      remove?: (options: { key: string }) => Promise<void>;
+    };
+  };
+}
+
+interface Window {
+  Capacitor?: CapacitorBridge;
+}