refactor: Consolidate navigation from Lit/Redux to React Router

[inureyes]

⚠️ Before implementing, read and follow the Implementation Rules in the epic issue #5364. These rules cover mandatory patterns for React component structure, BAI component usage, reactToWebComponent bridge, state management, i18n, error handling, styling, and code quality. The reference implementation is react/src/components/SignupModal.tsx.

Goal: Complete Lit file removal. Each migration must fully replace the Lit source file with React — not just the dialog parts. The migrated Lit .ts file and its type declarations must be deleted. This epic is a critical step toward removing all Lit/MWC/Vaadin dependencies from the codebase.

Description

The current navigation system is split between two frameworks:

• Lit/Redux (src/backend-ai-app.ts, src/components/backend-ai-webui.ts): installRouter() + Redux navigate/loadPage actions + dynamic page imports
• React Router v6 (react/src/routes.tsx, react/src/MainLayout.tsx): React Router with lazy-loaded page components

Bridge events currently connect them:

• move-to-from-react — React dispatches to trigger Lit navigation
• react-navigate — Lit dispatches to trigger React navigation
• locationPath:changed — Cross-framework location sync

This issue consolidates all routing into React Router and removes the Lit/Redux navigation layer.

Part of epic #5364
Depends on: #5371 (Login), #5366–#5368 (standalone views migrated so their Lit page loading can be removed)

Current Lit Navigation Architecture

src/backend-ai-app.ts (Redux actions):

navigate(path) → extract page from path → dispatch loadPage(page)
loadPage(page) → dynamic import(`./components/backend-ai-{page}.ts`) → update Redux store

src/components/backend-ai-webui.ts:

• installRouter() at line ~244 — captures URL changes
• stateChanged() at line ~846 — activates/deactivates page components based on Redux state
• _moveTo() at line ~808 — programmatic navigation with fromReact flag

Pages still loaded by Lit/Redux (not yet in React routes):

• verify-email → backend-ai-email-verification-view (migrating in feat: Migrate backend-ai-email-verification-view to React #5367)
• change-password → backend-ai-change-forgot-password-view (migrating in feat: Migrate backend-ai-change-forgot-password-view to React #5368)
• edu-applauncher → backend-ai-edu-applauncher (separate issue)
• Plugin pages → dynamically loaded (separate issue)
• Error/404 page → backend-ai-error-view

Migration Plan

1. Add missing React routes for all pages currently handled by Lit:
   • /verify-email → React EmailVerificationView (after feat: Migrate backend-ai-email-verification-view to React #5367)
   • /change-password → React ChangePasswordView (after feat: Migrate backend-ai-change-forgot-password-view to React #5368)
   • /edu-applauncher → React EduAppLauncher (separate issue)
   • /error → React error boundary page
2. Remove Redux page state (src/backend-ai-app.ts navigate/loadPage actions)
3. Remove installRouter() from backend-ai-webui.ts
4. Remove stateChanged() page activation logic from backend-ai-webui.ts
5. Remove bridge events (move-to-from-react, react-navigate) — no longer needed when all navigation is React Router
6. Remove _moveTo() method — replace remaining callers with React navigation
7. Remove backend-ai-app.ts if it only contains navigation logic

Acceptance Criteria

• Lit file removed: src/backend-ai-app.ts (if navigation-only) and its type declarations
• All page routes handled by React Router v6
• No Redux page state remaining
• installRouter() removed from app initialization
• Bridge events (move-to-from-react, react-navigate) removed
• _moveTo() method removed, callers updated
• 404/error page handled by React error boundary
• Browser back/forward navigation works correctly
• Deep linking works for all pages
• No navigation regressions