feat(dashboard): global (All Libraries) preferences scope

The dashboard's All Libraries view could render sections across every
library but could not persist its own preferences: the PUT required a
UUID library_id, and the prefs GET-without-library_id matched an
arbitrary library's row through a loose NULL OR-clause.

Storage: a dedicated user_dashboard_preferences_global table (one row
per user, user_id PK → users CASCADE) instead of a NULL/sentinel
library_id — chosen over both because (a) the column's FK to libraries
rules out a sentinel UUID without dropping the constraint or minting a
fake library row, and (b) intent becomes structural: only the explicit
global path writes that table, so a buggy or outside-the-API write can
never masquerade as global. System and user collections are
user-scoped, so the order/hidden arrays are library-independent and
valid in this scope.

API: PUT /api/dashboard/preferences binds library_id as *string —
JSON null/absent = global, empty string = 400 (a lost value must stay
loud, not silently clobber the global scope), valid UUID = per-library
(unchanged). GET without library_id now returns the global row instead
of an arbitrary one; the per-library query's dead NULL OR-arm is
removed. GET /sections keeps requiring library_id for per-library
views and aggregates across libraries when absent (unchanged).

Tests: global save/update-dedupe/read round-trip, empty-string 400,
global-vs-per-library independence. Two suite failures are the
handoff's documented pre-existing rot (duplicate system collections in
the dev DB; a sections test expecting a 400 the handler never
implemented) — reproduced and confirmed unrelated to this change.
Live-verified on the dev stack: null save 200 x2 with a single row
updated, empty string 400, global read carries the second save.

internal/services/dashboard_service.go

@@ -376,6 +376,22 @@ func (s *DashboardService) getUserCollectionItems(ctx context.Context, coll data
 }
 
 func (s *DashboardService) GetDashboardPreferences(ctx context.Context, userID uuid.UUID, libraryID pgtype.UUID) (database.UserDashboardPreferences, error) {
+	// Absent library = the global (All Libraries) scope: a separate
+	// one-row-per-user table, so an accidental per-library write can never
+	// masquerade as global.
+	if !libraryID.Valid {
+		g, err := s.db.GetGlobalDashboardPreferences(ctx, pgtype.UUID{Bytes: userID, Valid: true})
+		if err != nil {
+			return database.UserDashboardPreferences{}, err
+		}
+		return database.UserDashboardPreferences{
+			UserID:            g.UserID,
+			HiddenCollections: g.HiddenCollections,
+			CollectionOrder:   g.CollectionOrder,
+			ItemsPerSection:   g.ItemsPerSection,
+			UpdatedAt:         g.UpdatedAt,
+		}, nil
+	}
 	return s.db.GetDashboardPreferences(ctx, database.GetDashboardPreferencesParams{
 		UserID:    pgtype.UUID{Bytes: userID, Valid: true},
 		LibraryID: libraryID,
@@ -383,6 +399,24 @@ func (s *DashboardService) GetDashboardPreferences(ctx context.Context, userID u
 }
 
 func (s *DashboardService) UpsertDashboardPreferences(ctx context.Context, params database.UpsertDashboardPreferencesParams) (database.UserDashboardPreferences, error) {
+	if !params.LibraryID.Valid {
+		g, err := s.db.UpsertGlobalDashboardPreferences(ctx, database.UpsertGlobalDashboardPreferencesParams{
+			UserID:            params.UserID,
+			HiddenCollections: params.HiddenCollections,
+			CollectionOrder:   params.CollectionOrder,
+			ItemsPerSection:   params.ItemsPerSection,
+		})
+		if err != nil {
+			return database.UserDashboardPreferences{}, err
+		}
+		return database.UserDashboardPreferences{
+			UserID:            g.UserID,
+			HiddenCollections: g.HiddenCollections,
+			CollectionOrder:   g.CollectionOrder,
+			ItemsPerSection:   g.ItemsPerSection,
+			UpdatedAt:         g.UpdatedAt,
+		}, nil
+	}
 	return s.db.UpsertDashboardPreferences(ctx, params)
 }