fix: persist company filter across offer tabs

[0xgny]

Description ✏️

Closes #898

Persists the company filter when switching between the Internships and Full-Time
tabs, and adds a clearer empty state when the selected company has no offers of
that type.

• OffersNavigation now appends the current company param to both tab links. The
  logic is inside the component, so the desktop and mobile navs both get it.
• Only company is forwarded. hourlyRate is Internships-only, totalCompensation
  is Full-Time-only, and the location options are built per offer type, so
  forwarding those could filter a tab by a value missing from its own dropdown.
  page is dropped so you don't land on page 4 of a one-page list.
• Both list routes compute a specific emptyMessage: "No record exists for
  internship offers at {company} yet." It only shows when company is the sole
  active filter, since otherwise the empty result may be caused by the others.
• appliedCompany was already in both loader payloads, so no loader or query
  changes were needed.

Type of Change 🐞

• Feature - A non-breaking change which adds functionality.
• Fix - A non-breaking change which fixes an issue.
• Refactor - A change that neither fixes a bug nor adds a feature.
• Documentation - A change only to in-code or markdown documentation.
• Tests - A change that adds missing unit/integration tests.
• Chore - A change that is likely none of the above.

Checklist ✅

• I have done a self-review of my code.
• I have manually tested my code (if applicable).
• I have added/updated any relevant documentation (if applicable). => changes.md