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Adding availability for 5:30-6:00 PM with the lesson length left on its 60-minute default saved nothing and said nothing. A window is stored as consecutive lesson-length slots, so one that fits no lesson splits into none: splitByDuration() returned [], createFromWindow() inserted nothing, and addSlot() discarded the result and re-rendered the page unchanged. The REST endpoint already rejected that window with a 400. The admin form checked the same rules separately, and its copy was both laxer and mute — an unreadable date, an end before the start, and a two-day window were bare `return`s, and it never checked offering ownership at all, so a crafted POST could tie a slot to another instructor's offering and inherit their price and payment routing. Both callers now go through WindowValidator, which returns the window or a WP_Error explaining the refusal. The endpoint returns that error as is; the page renders its message as a notice. handleFormAction returns a [notice, error] pair so deletes report themselves too, and a successful add says how many slots it created. Two failures could also go unnoticed underneath: wpdb::insert's result was ignored, and insert_id still holds the previous statement's id after a failed write, so a failure looked like a success — and could become the recurrence group of a weekly series, orphaning every later occurrence. weeks was unbounded server-side despite the form's max=52. availability-admin.js narrows the lesson-length choices to those that fit the window and blocks submission when none do, which is what makes the original mistake hard to repeat. It is a convenience: the server validates regardless. Closes #130