Add a tsc/vue-tsc typecheck gate (8 pre-existing type errors in OfficeOverview.vue)

Discovered while setting up the Vitest harness (#47): there is no tsc/vue-tsc script anywhere in package.json — lint only runs eslint src. As a result, 8 pre-existing type errors in src/views/OfficeOverview.vue have never been caught by CI or local tooling.

Confirmed pre-existing and unrelated to the test-infra work: same errors, same count, with or without the OfficeOverview.vue/src/utils/ refactor from #47 (verified via git stash).

Running npx vue-tsc --noEmit on main currently reports:

• Node[] vs the object-literal shape returned by getAllOfficeFiles/resultToNode (TS2345, missing internal Node properties like _attributes/_data)
• INode not assignable to Node in several places (same root cause, template-side)
• a computed property name error (TS2464)
• Date | undefined not assignable to number | Date (NcDateTime's timestamp prop, x2)

Scope for this issue:

• Fix the 8 existing errors (likely centers on reconciling @nextcloud/files' Node vs INode types, and handling mtime: Date | undefined).
• Add a typecheck (or similar) script running vue-tsc --noEmit.
• Wire it into CI (new workflow or extend an existing one), separate from eslint.

Not bundling this into #47 — that issue is scoped to runtime test coverage (Vitest), not static type-checking, and this is an unrelated pre-existing gap.