feat(shows): block overlapping show times + drag day-view picker

[anneoneone]

Closes #163.

What

Blocks scheduling a show when its time window overlaps an existing show, and adds an Apple-Calendar-style day-view time picker.

Conflict blocking (both layers)

• Backend — AppError::Conflict (HTTP 409), pure time_windows_overlap helper, and ensure_no_show_conflict (queries same-date shows, excludes self on edit). Wired into api_create_show and api_update_show. Half-open [start, end) overlap; missing end = zero-length point; identical starts always clash. 4 unit tests.
• Frontend — showConflict.ts (rangesOverlap / findConflictingShow) mirrors backend semantics. The wizard date step loads all scheduled shows, exposes conflictingShow, and gates canProceed. 9 Vitest cases.

Day-view picker (DayTimeline.vue)

• Hour grid beside the month calendar. Click an empty slot → default 1-hour show; drag top/bottom handles to resize, body to move (15-min snap).
• Existing shows render as read-only blocks; the placed block turns red on conflict.
• Height tracks the calendar via a ResizeObserver so both columns match.

UX

• After creation, navigate to the new show's detail page (/shows/:id) for hosts and admins, instead of the overview / stream.

Verification

• Backend: cargo fmt + clippy clean, 4/4 overlap tests pass.
• Frontend: 31/31 Vitest, ESLint clean, no new tsc errors (pre-existing .vue-resolution errors only).
• Drag interaction needs a manual browser check (no real layout in jsdom).