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/handlers/dashboard.go

@@ -75,7 +75,9 @@ func (h *DashboardHandler) UpdatePreferences(c *echo.Context) error {
 	userUUID := uuid.UUID(user.ID.Bytes)
 
 	var req struct {
-		LibraryID         string   `json:"library_id"`
+		// nil (JSON null/absent) = the global All Libraries scope;
+		// empty string is a LOST value, not a scope — rejected loudly.
+		LibraryID         *string  `json:"library_id"`
 		HiddenCollections []string `json:"hidden_collections"`
 		CollectionOrder   []string `json:"collection_order"`
 		ItemsPerSection   int      `json:"items_per_section"`
@@ -85,9 +87,16 @@ func (h *DashboardHandler) UpdatePreferences(c *echo.Context) error {
 		return c.JSON(http.StatusBadRequest, map[string]string{"error": "Invalid request"})
 	}
 
-	libUUID, err := uuid.Parse(req.LibraryID)
-	if err != nil {
-		return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid library_id"})
+	var libUUID pgtype.UUID
+	if req.LibraryID != nil {
+		if *req.LibraryID == "" {
+			return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid library_id"})
+		}
+		parsed, err := uuid.Parse(*req.LibraryID)
+		if err != nil {
+			return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid library_id"})
+		}
+		libUUID = pgtype.UUID{Bytes: parsed, Valid: true}
 	}
 
 	// Sanitize preferences (remove duplicates)
@@ -97,7 +106,7 @@ func (h *DashboardHandler) UpdatePreferences(c *echo.Context) error {
 
 	prefs, err := h.dashboardService.UpsertDashboardPreferences(c.Request().Context(), database.UpsertDashboardPreferencesParams{
 		UserID:            pgtype.UUID{Bytes: userUUID, Valid: true},
-		LibraryID:         pgtype.UUID{Bytes: libUUID, Valid: true},
+		LibraryID:         libUUID,
 		HiddenCollections: req.HiddenCollections,
 		CollectionOrder:   req.CollectionOrder,
 		ItemsPerSection:   pgtype.Int4{Int32: int32(req.ItemsPerSection), Valid: true},