Filter booking calendar slots by available lesson type

Not every open time can be booked as every private-lesson type: a slot tied
to an offering takes that offering only, and a generic slot only takes types
whose length fits. Students had no way to see that before clicking a time.

The booking calendar now carries a lesson-type filter — a checkbox per active
private-lesson type, fetched once from GET /offerings?kind=private_lesson.
Ticking types narrows the calendar to the times bookable as one of them and
re-anchors the week view on the earliest match. The registration form's
Lesson type picker is narrowed the same way, and a lone remaining type is
pre-selected with its intake questions loaded.

Bookability is decided by offeringFitsSlot(), the client-side mirror of the
rule POST /bookings enforces; the filter is a browsing aid and the server
still validates every booking. No ticks means no filter, and the whole
control is hidden when the studio offers fewer than two private-lesson types.

Closes #117

Co-Authored-By: Claude Opus 5 <[email protected]>

CHANGELOG.md

@@ -14,6 +14,7 @@ each change under the current top section as you work.
 ## [1.2.2]
 
 ### Added
+- The booking calendar now has a **lesson type** filter above it, so a student browsing open times can narrow them to the types they actually want. Because not every open time can be booked as every private-lesson type — some times are tied to a specific type, others only take types of a matching length — the filter shows just the times bookable as the ticked types, and re-anchors the week view on the earliest one so it never opens on an empty week. Picking one of those times narrows the **Lesson type** picker on the registration form to the same list, and when only one type is left it is chosen automatically with its questions loaded. Tick nothing (or use **Show all types**) to see every open time as before. The filter is hidden when the studio only offers one private-lesson type.
 - The **Group Classes** block can now be pinned to a single class, under **Classes shown → Class** in the block sidebar (shortcode: `[us_group_classes offering="…"]`). Pick a class and the block shows only that one, so it can be embedded on a page that describes the class. In this mode the class's own description is left out to avoid repeating the page copy — the card shows the schedule, instructor, price, enrolment deadline and the enrol/withdraw controls. Leaving it on **All classes** keeps the full browsable catalog with descriptions.
 - The **Student Registration** block can now send students onward to a page of your choosing once they finish registering. Its **After email confirmation** panel is now **After registration**: the page you pick there is where the link shown to a newly registered student points — the "Sign in to your account" link after they confirm their email, and a "Continue to your account" link for an invited student, who is signed in immediately. A new **Redirect automatically** option takes them straight there instead of showing the link. Registration errors are never skipped — a failed sign-up and an expired confirmation link still show their message on the page, as does the "check your email to confirm your address" step. The redirect needs a page to be chosen; with none set, students see the link (or, for invited students, just the confirmation) as before.