Add pull-to-refresh to the profile screen

**Problem**

Candidate data is fetched once per app launch, cached in a global, and cleared only on logout. The profile screen has no refresh affordance.

Applying to a company deducts `applicationCost` from the student's credit pool server-side, but the profile keeps showing the old balance until the app is force-killed.

**Where**

- `lib/services/generic/applyService.dart:43-64` (cache short-circuit at :44)
- `lib/shared/GlobalCache.dart:15`
- `lib/views/CandidateDetailsView.dart:33-42` (no `RefreshIndicator`)
- Server-side deduction: `omniport-app-placement-and-internship/views/application.py:159-163`

**Expected**

Pull to refresh re-fetches the candidate, including the credit balance.

**Fix sketch**

- Add a `refreshAndWait` to `CandidateDetailsViewModel` that nulls the cached candidate and refetches, mirroring `ProfilesForMeViewModel`.
- Wrap the body in a `RefreshIndicator`.

Start with lib/views/CandidateDetailsView.dart:33-42 and locate CandidateDetailsViewModel, then compare its fetching flow with ProfilesForMeViewModel. Read the cache short-circuit in lib/services/generic/applyService.dart:43-64 and lib/shared/GlobalCache.dart:15. Done means pulling on the profile refetches candidate data and displays the updated credit balance.

Written by the indexing model from the issue text.