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/queries/queries.sql

@@ -2435,9 +2435,13 @@ LIMIT $1 OFFSET $2;
 -- ============================================
 
 -- 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.
 -- name: GetDashboardPreferences :one
 SELECT * FROM user_dashboard_preferences
-WHERE user_id = sqlc.narg('user_id') AND (sqlc.narg('library_id')::uuid IS NULL OR library_id = sqlc.narg('library_id')::uuid);
+WHERE user_id = sqlc.narg('user_id') AND library_id = sqlc.narg('library_id')::uuid;
 
 -- name: UpsertDashboardPreferences :one
 INSERT INTO user_dashboard_preferences (user_id, library_id, hidden_collections, collection_order, items_per_section)
@@ -2459,6 +2463,22 @@ SET hidden_collections = $2,
 WHERE user_id = $1 AND library_id = $5
 RETURNING *;
 
+-- Global (all-libraries) scope: exactly one row per user.
+-- name: GetGlobalDashboardPreferences :one
+SELECT user_id, hidden_collections, collection_order, items_per_section, updated_at FROM user_dashboard_preferences_global
+WHERE user_id = $1;
+
+-- name: UpsertGlobalDashboardPreferences :one
+INSERT INTO user_dashboard_preferences_global (user_id, hidden_collections, collection_order, items_per_section)
+VALUES ($1, $2, $3, $4)
+ON CONFLICT (user_id)
+DO UPDATE SET
+    hidden_collections = EXCLUDED.hidden_collections,
+    collection_order = EXCLUDED.collection_order,
+    items_per_section = EXCLUDED.items_per_section,
+    updated_at = NOW()
+RETURNING user_id, hidden_collections, collection_order, items_per_section, updated_at;
+
 -- Dashboard collections queries
 -- name: GetSystemCollectionsForDashboard :many
 SELECT * FROM collections