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/database/querier.go

@@ -174,6 +174,10 @@ type Querier interface {
 	// CAROUSEL-STYLE DASHBOARD
 	// ============================================
 	// Dashboard preferences queries
+	// The handler routes by scope: a Valid library_id targets the per-library
+	// table below; an absent one targets user_dashboard_preferences_global
+	// (the "All Libraries" view). Never pass NULL here — the loose NULL match
+	// was removed because it returned an arbitrary library's row.
 	GetDashboardPreferences(ctx context.Context, arg GetDashboardPreferencesParams) (UserDashboardPreferences, error)
 	GetDevice(ctx context.Context, id pgtype.UUID) (Devices, error)
 	GetDeviceByAuthToken(ctx context.Context, authToken string) (Devices, error)
@@ -200,6 +204,8 @@ type Querier interface {
 	GetFailedSyncQueueItems(ctx context.Context, limit int32) ([]SyncQueue, error)
 	GetFirstAdmin(ctx context.Context) (pgtype.UUID, error)
 	GetFirstAdminExclude(ctx context.Context, id pgtype.UUID) (GetFirstAdminExcludeRow, error)
+	// Global (all-libraries) scope: exactly one row per user.
+	GetGlobalDashboardPreferences(ctx context.Context, userID pgtype.UUID) (UserDashboardPreferencesGlobal, error)
 	GetHashConflict(ctx context.Context, id pgtype.UUID) (HashConflicts, error)
 	GetKoboEntitlementByContentId(ctx context.Context, arg GetKoboEntitlementByContentIdParams) (GetKoboEntitlementByContentIdRow, error)
 	GetKoboEntitlementByEntitlementId(ctx context.Context, arg GetKoboEntitlementByEntitlementIdParams) (GetKoboEntitlementByEntitlementIdRow, error)
@@ -503,6 +509,7 @@ type Querier interface {
 	UpdateUserTimezone(ctx context.Context, arg UpdateUserTimezoneParams) error
 	UpdateUsername(ctx context.Context, arg UpdateUsernameParams) error
 	UpsertDashboardPreferences(ctx context.Context, arg UpsertDashboardPreferencesParams) (UserDashboardPreferences, error)
+	UpsertGlobalDashboardPreferences(ctx context.Context, arg UpsertGlobalDashboardPreferencesParams) (UserDashboardPreferencesGlobal, error)
 	UpsertPanelData(ctx context.Context, arg UpsertPanelDataParams) (PanelData, error)
 	UpsertReaderSettings(ctx context.Context, arg UpsertReaderSettingsParams) (ReaderSettings, error)
 	UpsertSystemSetting(ctx context.Context, arg UpsertSystemSettingParams) (SystemSettings, error)