fix: correct semester numbering and rework the semester/week picker

[HeZeBang]

• Semester term numbers sent to /schedule/term_begin were derived from a 春/秋-substring check that silently collapsed 秋 and 暑 (and any raw digit-keyed term) onto "2"; switch to a shared 秋=1/春=2/暑=3 mapping used consistently for both display and the API request.
• Replace the flat semester list (mobile bottom sheet, iOS native submenu, desktop popover) with a synced year/term scroll-wheel picker shared across all three surfaces.
• Consume the full /schedule/term_begin response (teaching-week count, class-period counts, etc.) instead of just the term-begin date, so the week picker/pager sizes itself to the semester's actual teaching-week count and hides the "this week"/"today" indicators when today falls outside the selected semester.
• Swap the hand-rolled fade+slide week-number transition for the reel_text package's roll animation.