feat(ui): redesign native study workspace

[SebastianBoehler]

Summary

• replace the card-heavy shell with a native, focused study workspace
• simplify home, schedule, inbox, Mensa, settings, and assistant surfaces
• replace the notched floating Ask control with a stable Assistant tab

Validation

• flutter analyze
• flutter test

[SebastianBoehler]

Review pass on the draft UI redesign: the intent is clear—move from the dark/card-heavy shell to a lighter native workspace, replace the drawer/FAB with More + Assistant navigation, and simplify Home/Schedule/Mail/Mensa/chat surfaces.

Validation is in good shape: formatting, flutter analyze, and flutter test pass. I found no security/privacy or persistence-contract regressions. The remaining risk is mostly visual/navigation/accessibility behavior:

• schedule_components.dart:163-167: the first event in any selected day is labeled Next, even if that day/event is in the past. Please derive “Next” from the actual next upcoming event (or suppress it for ended events) and cover a past-day/all-events-ended case.
• studyos_theme.dart:91-109 + composer_bar.dart:57-63: the new global enabledBorder/focusedBorder can override the composer's border: InputBorder.none, producing an inner outlined/filled field inside the already bordered composer. Please explicitly clear the state borders/fill there and visually verify focused + unfocused states.
• The redesign newly leaves secondary_destinations_drawer.dart and generated_ui_preview_section.dart without production references. If their removal is intentional, deleting them keeps the old UI path from becoming dead code.
• Useful focused review/test angles: announce the selected day in ScheduleDayStrip semantics, and exercise More-menu routing plus the new chat-history modal. These are non-blocking test gaps, but they cover the main changed navigation contracts.

Smallest next step: fix the two visible behavior issues above, remove/justify the orphaned widgets, then let the queued platform builds finish and do one narrow mobile visual pass (small width + larger text scale). No merge action taken.