feat: profile sharing via X25519 envelope (Phase B, #3)

An owner can now share a profile with another account; the recipient
reads the profile's metadata AND its data (medications, appointments,
health stats) under their own login. The server stays a blind store:
sharing uses an X25519 envelope — the owner wraps the profile DEK to
the recipient's identity public key via ECDH (fresh ephemeral key per
share), the recipient unwraps it with their identity private key.

Backend:
- models/profile_share.rs: ProfileShare + ProfileShareRepository
  (find_for_recipient, find_for_profile, find_active [checks active +
  expiry], delete, upsert). Indexed on (profileId, recipientUserId) and
  recipientUserId.
- handlers/profile_share.rs: POST/GET /profiles/:id/shares,
  DELETE /profiles/:id/shares/:recipient, GET /profiles/shared-with-me,
  GET /users/public-key (public), and authorize_profile_read — the
  share-gate that admits owner OR active-share recipient.
- The share-gate is wired into list/get for medications, appointments,
  and health stats: when profile_id is specified, resolve the owner via
  the gate and query as them. Data repos stay ownership-scoped.
- Removed the legacy Share system (ADR Open Q5): models/{share,
  permission}.rs, handlers/{shares,permissions}.rs, middleware/
  permission.rs (dead), the shares collection field + methods in
  mongodb_impl.rs, the shares index, and the 5 /api/shares +
  /api/permissions routes.
- share_tests.rs: full owner→recipient→revoke flow, ownership
  isolation, share-to-self/nonexistent/keyless rejections, expired
  share treated as absent.

Frontend:
- crypto/keys.ts: wrapProfileDekToRecipient /
  unwrapProfileDekFromShare (ECDH envelope, ephemeral key per share).
- useProfileStore: loadSharedWithMe (unwrap each share's DEK with the
  identity private key), shareProfile, revokeShare, loadProfileShares.
  Shared profiles merge into the list with is_shared=true.
- ProfileSwitcher: shows shared profiles with a 'shared' chip.
- ProfileSharing (new) + ProfileEditor: owner UI to add a recipient by
  email and revoke; shared profiles render read-only with owner info.
- ECDH round-trip test (owner wraps, recipient unwraps, stranger can't).

Verification: backend cargo build/clippy (-D warnings)/fmt clean, tests
compile (integration tests run in CI — Mongo is fixed there). Frontend
tsc clean, 31/31 tests pass.

⚠️ Backend integration tests not run locally (no Mongo/Docker in this
sandbox); CI will run them — I'll fix any failures immediately.

Phases C (hard revoke / re-key) and D (graduation) remain. Refs #3.

backend/src/handlers/health_stats.rs

@@ -90,8 +90,20 @@ pub async fn list_health_stats(
                 .into_response()
         }
     };
+    // Share-gate: if a profile_id is specified, resolve the owner (admits the
+    // owner or an active share recipient) and query as them.
+    let owner = match &query.profile_id {
+        Some(pid) => {
+            match crate::handlers::profile_share::authorize_profile_read(&state, &claims, pid).await
+            {
+                Ok(o) => o,
+                Err(resp) => return resp.into_response(),
+            }
+        }
+        None => claims.sub.clone(),
+    };
     match repo
-        .find_by_user_filtered(&claims.sub, query.profile_id.as_deref())
+        .find_by_user_filtered(&owner, query.profile_id.as_deref())
         .await
     {
         Ok(stats) => {
@@ -134,10 +146,17 @@ pub async fn get_health_stat(
 
     match repo.find_by_id(&object_id).await {
         Ok(Some(stat)) => {
-            if stat.user_id != claims.sub {
-                return (StatusCode::FORBIDDEN, "Access denied").into_response();
+            // Share-gate on the stat's profile (admits owner or recipient).
+            match crate::handlers::profile_share::authorize_profile_read(
+                &state,
+                &claims,
+                &stat.profile_id,
+            )
+            .await
+            {
+                Ok(_) => (StatusCode::OK, Json(HealthStatResponse::from(stat))).into_response(),
+                Err(resp) => resp.into_response(),
             }
-            (StatusCode::OK, Json(HealthStatResponse::from(stat))).into_response()
         }
         Ok(None) => (
             StatusCode::NOT_FOUND,