Split job offer detail into a separate page

[KaliCZ]

Problem

The list and detail views for job offers are crammed into a single page (job-offers.astro and admin/job-offers.astro). This causes:

• Flash-of-wrong-content (filter/list briefly visible before detail loads)
• Complex visibility toggle logic (hidden class juggling between sections)
• Race conditions between list and detail rendering
• Hash-based navigation (#id) instead of proper URLs
• Each page file is huge with two views worth of code

Proposed change

Split the detail view into a dedicated page using a query param for the ID:

• /job-offers/detail?id=... — user detail page
• /admin/job-offers/detail?id=... — admin detail page

These are still static Astro pages (no SSR needed) since all content is client-rendered after auth anyway. The ID is read from URLSearchParams instead of location.hash.

Benefits

• Each page renders exactly one view — no visibility juggling
• Direct, shareable URLs without hash fragments
• Roughly half the JS per page
• No flash between list and detail states
• JobOfferDetail.astro component is already extracted — it just needs its own page wrapper

Migration

• List pages link to /job-offers/detail?id=... instead of setting hash
• Detail pages read id from query params
• Back button navigates to /job-offers (or /admin/job-offers)
• Remove hash-based navigation, popstate listeners, and visibility toggles from list pages

Files

• frontend/src/pages/[...lang]/job-offers.astro — strip detail code, link to detail page
• frontend/src/pages/[...lang]/admin/job-offers.astro — same
• frontend/src/pages/[...lang]/job-offers/detail.astro — new, user detail page
• frontend/src/pages/[...lang]/admin/job-offers/detail.astro — new, admin detail page
• frontend/src/components/JobOfferDetail.astro — already exists, reused as-is