Extract a shared reactive Collection-view helper — 5x duplicated Yjs observer boilerplate

[brylie]

Problem

TableCollectionView.svelte, BoardCollectionView.svelte, CalendarCollectionView.svelte, FieldManagerDialog.svelte, and /table/[id]/+page.svelte each independently re-implement the same boilerplate for reading a live Collection out of Yjs:

1. Call getCollectionView(doc, collectionId) (or getCollection) to get the current snapshot.
2. Mirror pieces of it (schema, rows, and — since Make the Collection primary field and Board card title explicit #96 — primaryFieldKey) into local $state variables.
3. Subscribe collectionsMap.observeDeep/recordsMap.observeDeep in onMount to re-run step 1–2 on every Yjs change.
4. Unsubscribe on unmount.

This predates #96, but implementing #96's primaryFieldKey meant touching all five copies again to add one more mirrored field. Each copy is a small but exact duplicate of the others (see src/lib/components/BoardCollectionView.svelte's refresh()/onMount for the canonical shape), which is exactly the kind of drift risk this repo's service-layer/shared-projection conventions (docs/specifications/collection-views.md §3, service-layer.md) already try to avoid for other cross-cutting concerns.

Scope

• Extract a shared reactive helper (e.g. useCollectionView(collectionId: () => string) in src/lib/data/ or src/lib/client/, using Svelte 5 runes) that encapsulates: initial read, Yjs observer subscription/cleanup, and exposes { schema, rows, primaryFieldKey, collection } as reactive state.
• Migrate all five existing call sites (TableCollectionView, BoardCollectionView, CalendarCollectionView, FieldManagerDialog, /table/[id]/+page.svelte) to use it, deleting their individual refresh()/onMount observer boilerplate.
• No behavior change — existing component tests should pass unmodified (or with only import/setup changes, not assertion changes).

Non-goals

• Not a broader state-management rewrite (no store library, no global cache) — just deduplicating this one specific pattern.
• Not changing what data is exposed (still schema/rows/primaryFieldKey) — a future PR can extend the helper if a sixth derived field shows up.

Done when

There is exactly one implementation of "read a Collection reactively from Yjs and keep it live," all five current call sites use it, and the existing test suite passes with no behavior change.

[brylie]

Set Priority: P2.

Real tech-debt/maintainability value (one implementation instead of five duplicated copies), but no user-facing urgency and nothing else in the backlog depends on it landing first.