Use shadcn Button component for save card and scheduling bar buttons

[zhiganov]

Problem

Two components use raw <button> elements instead of the shadcn Button component, creating visual inconsistency with the rest of the poll page:

1. Save card buttons (PollView.jsx:548-550, 563-565)

The "Save availability" and "Save changes" buttons in the sidebar save card are hand-styled <button> elements.

2. Scheduling bar buttons (SchedulingGrid.jsx:137-149)

The Cancel and Schedule buttons in the scheduling mode bar are raw <button> elements.

Solution

Replace with <Button> from @/components/ui/button using appropriate variants.

[zhiganov]

Partially addressed: save card buttons now use <Button> (from sticky bar work in ui-audit-fixes). Scheduling bar Cancel/Confirm buttons still use raw <button>.