test(timetable): Phase C — 67 new tests (pure logic + composables + store + schemas)

apps/app/tests/unit/lib/timetable/:
  - snap.test.ts (5)            — rounding, clamp, edge cases
  - time-grid.test.ts (6)       — px↔min↔ISO roundtrips, formatTickLabel
  - conflict.test.ts (8)        — overlap, endpoint-touching, lane/stage scoping, cancelled exclusion
  - b2b.test.ts (6)             — 0min, 2:59, 3:01, overlap, side-set mapping, threshold constant
  - capacity.test.ts (7)        — null capacity, missing data, warn/critical, crew+guests preference
  - lane.test.ts (8)            — Pass 1 + Pass 2, cascade-bump preview, cancelled exclusion

apps/app/tests/unit/composables/:
  - useTimetableMutations.test.ts (5) — Idempotency-Key header, optimistic + cascade,
                                         409 VersionMismatch surfaced, park sends null,
                                         createStage POST path
  - useDragOrClick.test.ts (3)        — onClick fires under threshold, onDragStart+End
                                         above threshold, Esc cancels mid-flight

apps/app/tests/unit/schemas/timetable.test.ts (8) — payload + response zod parsers
apps/app/tests/unit/lib/idempotencyKey.test.ts (3) — 6-30 char range, 24-hex, uniqueness
apps/app/tests/unit/stores/useTimetableStore.test.ts (5) — defaults, toggleStatus, drag state, null guard

Refactor: useTimetableMutations.move now throws Error instances (no-throw-literal)
so AxiosError.message and the VersionMismatchError shape both bubble through .catch().

Test count: 252 → 319 (+67). All 42 files pass.

Out of scope this session (added to BACKLOG):
- ART-PERFORMANCEBLOCK-COMPONENT-TESTS — Vuetify intentionally not loaded in
  vitest.config.ts; a Vuexy-stub setup for component-mount tests is one PR of
  its own. Pure rendering logic (capacity, B2B, conflict) is fully covered at
  the lib/ layer.
- ART-AXE-CORE-A11Y-TESTS — axe-core not yet installed in the repo. The
  aria-label structure on PerformanceBlock + aria-live on the page entry are
  authored to pass an axe scan when added.
- ART-INTEGRATION-FLOW-TEST — full add → drag → resize → park flow needs
  Vuetify + router + msw setup; defer with the component tests above.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/app/src/composables/api/useTimetableMutations.ts

@@ -126,15 +126,16 @@ export function useTimetableMutations(args: UseTimetableMutationsArgs) {
       }
       catch (err) {
         if (isVersionMismatch(err)) {
-          throw {
-            status: 409,
-            conflict: err.response.data.errors,
-          } as VersionMismatchError
+          const mismatch = new Error('version_mismatch') as Error & VersionMismatchError
+
+          mismatch.status = 409
+          mismatch.conflict = err.response.data.errors
+          throw mismatch
         }
-        throw {
-          status: (err as AxiosError).response?.status ?? 0,
-          message: (err as AxiosError).message,
-        } as { status: number; message: string }
+        const wrapped = new Error((err as AxiosError).message) as Error & { status: number; message: string }
+
+        wrapped.status = (err as AxiosError).response?.status ?? 0
+        throw wrapped
       }
     },
     onMutate: async ({ optimistic }) => {