feat: per-profile DEKs + multi-profile (Phase A2, #3)

Implements the 3-tier key model from the multi-person sharing ADR:
each account owns multiple profiles (a person or pet — a 'subject of
care'), and each profile has its own random AES-256-GCM DEK. All
health data is now encrypted under the active profile's DEK, not the
account-wide DEK. The account DEK wraps each profile DEK; the server
stores only opaque wrapped blobs.

Per the ADR: DB wipe, no migration (no real user data). This unblocks
Phase B (sharing) — there is now a per-profile key to wrap to a
recipient's X25519 public key.

Backend:
- Profile model: owner_account_id, kind (human/pet), relationship,
  wrapped_profile_dek + iv. ProfileRepository gains find_all_by_owner,
  find_by_profile_id_owned, update_profile, delete_profile — all
  ownership-scoped.
- Profile handlers: GET/POST /api/profiles, GET/PUT/DELETE
  /api/profiles/:id. Removed /api/profiles/me. Renamed users.rs
  get_profile/update_profile (the /api/users/me handlers) to
  get_account/update_account to resolve a name collision.
- Register accepts default_profile_* fields and auto-creates the self
  profile when the client provides a wrapped profile DEK.
- HealthStatistic + Appointment gain profile_id and ?profile_id=
  filtering (health stats previously had no profile binding).
- New profile_tests.rs: multi-profile CRUD + ownership isolation +
  register-with-default-profile. Fixed the zk health-stat test to
  send the now-required profile_id.

Frontend:
- crypto/keys.ts: generateProfileDek, wrapProfileDek, unwrapProfileDek
  + in-memory per-profile DEK store with an active-profile concept.
- useProfileStore rewritten: holds profiles[], activeProfileId;
  loadProfiles unwraps each profile DEK; create/update/delete. All 11
  encrypt/decrypt sites switched from getEncKey() to
  getActiveProfileDek(). load actions pass ?profile_id= so only the
  active profile's rows come back.
- ProfileEditor rewritten for the new store (edit active profile,
  create/delete). New ProfileSwitcher in the Dashboard AppBar.
- MedicationManager / AppointmentsManager use the active profile id
  instead of the hardcoded profile_<user_id>.
- 2 new crypto tests for per-profile DEK isolation; updated store +
  component tests for the active-profile-DEK model.

Verification: backend cargo build/clippy/fmt green, tests compile
(integration tests run in CI — Mongo is fixed there). Frontend
tsc clean, 30/30 tests pass.

Closes nothing yet (Phase B/C/D remain). Refs #3.

backend/tests/zk_integration_tests.rs

@@ -179,6 +179,7 @@ async fn health_stat_stored_as_ciphertext() {
         "POST",
         "/api/health-stats",
         Some(json!({
+            "profile_id": "default",
             "encrypted_data": { "data": opaque_data, "iv": "aXY=" },
             "recorded_at": "2026-07-01T10:00:00Z",
         })),