[Frontend]: Replace placeholder pages with data-driven implementations

[Hanseooo]

Problem

Core authenticated pages still render placeholder/static states and are not fully connected to backend data/actions.

Context

• Reference: docs/FUTURE_ISSUES_BACKLOG.md
• Affected pages:
  • frontend/atla/src/pages/HomePage.tsx
  • frontend/atla/src/pages/TripsPage.tsx
  • frontend/atla/src/pages/ExplorePage.tsx
  • frontend/atla/src/pages/ProfilePage.tsx

Requirements

• Connect pages to live API hooks/queries.
• Implement key actions: view trips, navigate to trip detail, profile update flow, and explore search results.
• Add consistent loading, empty, and error states.
• Keep auth guards and route behavior intact on desktop/mobile layouts.

Acceptance Criteria

• Pages render live data (not static placeholder copy) using current backend APIs.
• Main CTA flows are functional and navigable.
• Error and empty states are user-friendly and consistent.
• Typecheck/build pass and no route guard regressions.

Dependencies

• Depends on [Backend]: Persist chat-generated itineraries to trips tables #37 and [Backend]: Implement /api/trips endpoints for list and detail #38 for trip data availability.
• Related to [Backend]: Wire chat auth dependency to real user identity #39 for user-linked behavior consistency.

Out of Scope

• New product features unrelated to replacing placeholder data