Collections sidebar clutters with empty collections the user has no visible nodes in #1487

Summary

The Collections sidebar lists every collection in the tenant, including ones the signed-in user has zero visible member nodes in. On tenant_demo a user invited to a single collection ("T4 Collection") sees ~17 additional collections (Skills, Strategy, Architecture, Development, References, Benchmarks, Components, Standards, Docs, .claude, Decisions, Research, Go To Market, Alternatives, Onboarding, Github Issue Management, Market Validation) that are all empty — clicking any of them shows "No nodes in this collection". This clutters the tree and makes it hard to find the one collection that actually has content.

Repro

1. Sign in to a tenant where your identity is a member of only one collection.
2. Open the Collections tree in the sidebar.
3. Observe: the single populated collection is expandable, but a long list of empty collections is rendered alongside it. Each opens to an empty sub-panel.

Expected

Collections with no member nodes visible to the current user should be hidden by default (or visually de-emphasised / collapsed under a "show empty" toggle). Only collections that have content the user can see should appear in the primary list.

Notes / pointers

• Sidebar tree: packages/desktop-app/src/lib/components/layout/navigation-sidebar.svelte
• Sub-panel that renders No nodes in this collection: packages/desktop-app/src/lib/components/layout/collection-sub-panel.svelte:51
• Collections store loads all collections unconditionally: packages/desktop-app/src/lib/stores/collections.ts (getAllCollections() → no empty filter).
• Decision needed: hide-empty is a UI filter, but "empty for THIS user" depends on member visibility (RBAC), so the filter should key off the member count the store already fetches per collection, not a global node count.

Found while validating Google sign-in (#11) on a freshly-provisioned tenant_demo user.