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/models.go

@@ -538,6 +538,14 @@ type UserDashboardPreferences struct {
 	UpdatedAt         pgtype.Timestamptz `db:"updated_at" json:"updated_at"`
 }
 
+type UserDashboardPreferencesGlobal struct {
+	UserID            pgtype.UUID        `db:"user_id" json:"user_id"`
+	HiddenCollections []string           `db:"hidden_collections" json:"hidden_collections"`
+	CollectionOrder   []string           `db:"collection_order" json:"collection_order"`
+	ItemsPerSection   pgtype.Int4        `db:"items_per_section" json:"items_per_section"`
+	UpdatedAt         pgtype.Timestamptz `db:"updated_at" json:"updated_at"`
+}
+
 type Users struct {
 	ID           pgtype.UUID        `db:"id" json:"id"`
 	Email        string             `db:"email" json:"email"`