fix(visits): anchor the notable window at the next visit's preparation

Without from, GET /api/day/notable started at the last completed visit
of any kind, while the web passes the next planned visit's
preparation.from, so a client that omits from read a different list for
the same appointment. Without from the window now starts at that
preparation day (anchor nextVisit, documented in OpenAPI); with nothing
booked it keeps the last completed visit or the 90-day fallback.

src/app/api/day/notable/route.ts

@@ -1,8 +1,9 @@
 /**
  * `GET /api/day/notable?from=&to=` — what changed and what stood out in a
  * window, for the preparation of the next visit (v1.42, #613). Without
- * `from` the window starts at the last completed visit the caller may see;
- * without `to` it ends today. The shape is `DayNotableWindowResponse` in
+ * `from` the window is the one the next planned visit prepares (its
+ * `preparation.from`), else it starts at the last completed visit the caller
+ * may see; without `to` it ends today. The shape is `DayNotableWindowResponse` in
  * `src/lib/day/contract.ts`.
  *
  * Observations and changes only, as keys and the record's own names: no