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.

cmd/server/tests/dashboard_integration_test.go

@@ -258,6 +258,151 @@ func (s *DashboardIntegrationTestSuite) TestRestoreSystemCollection_ValidNames()
 	}
 }
 
+func (s *DashboardIntegrationTestSuite) TestUpdatePreferences_GlobalScope() {
+	token := s.setup.Token
+
+	// Absent library_id (JSON null after omit) = the global All Libraries
+	// scope: stored in user_dashboard_preferences_global, one row per user.
+	reqBody := map[string]interface{}{
+		"hidden_collections": []string{"not-started"},
+		"collection_order":   []string{"continue-reading", "recently-added"},
+		"items_per_section":  15,
+	}
+	body, _ := json.Marshal(reqBody)
+	req, _ := http.NewRequest("PUT", s.setup.Server.URL+"/api/dashboard/preferences", bytes.NewBuffer(body))
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("Authorization", "Bearer "+token)
+
+	client := &http.Client{}
+	resp, err := client.Do(req)
+	require.NoError(s.T(), err)
+	defer func(Body io.ReadCloser) {
+		_ = Body.Close()
+	}(resp.Body)
+
+	assert.Equal(s.T(), http.StatusOK, resp.StatusCode)
+
+	// Second global save with different values: must UPDATE the same row
+	// (the old ON CONFLICT (user_id, library_id) never fired for NULLs and
+	// would have minted a duplicate per save).
+	reqBody["items_per_section"] = 25
+	body, _ = json.Marshal(reqBody)
+	req, _ = http.NewRequest("PUT", s.setup.Server.URL+"/api/dashboard/preferences", bytes.NewBuffer(body))
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("Authorization", "Bearer "+token)
+	resp, err = client.Do(req)
+	require.NoError(s.T(), err)
+	defer func(Body io.ReadCloser) {
+		_ = Body.Close()
+	}(resp.Body)
+	assert.Equal(s.T(), http.StatusOK, resp.StatusCode)
+
+	// Reading prefs without library_id returns the global row (not some
+	// arbitrary library's).
+	req, _ = http.NewRequest("GET", s.setup.Server.URL+"/api/dashboard/preferences", nil)
+	req.Header.Set("Authorization", "Bearer "+token)
+	resp, err = client.Do(req)
+	require.NoError(s.T(), err)
+	defer func(Body io.ReadCloser) {
+		_ = Body.Close()
+	}(resp.Body)
+	assert.Equal(s.T(), http.StatusOK, resp.StatusCode)
+
+	var prefs map[string]interface{}
+	err = json.NewDecoder(resp.Body).Decode(&prefs)
+	require.NoError(s.T(), err)
+	assert.Equal(s.T(), float64(25), prefs["items_per_section"], "global row should carry the second save")
+}
+
+func (s *DashboardIntegrationTestSuite) TestUpdatePreferences_EmptyLibraryIDRejected() {
+	token := s.setup.Token
+
+	// Empty string is a LOST value (a client bug), never a scope — must
+	// stay a loud 400 rather than silently writing the global scope.
+	reqBody := map[string]interface{}{
+		"library_id":         "",
+		"hidden_collections": []string{},
+		"collection_order":   []string{},
+		"items_per_section":  20,
+	}
+	body, _ := json.Marshal(reqBody)
+	req, _ := http.NewRequest("PUT", s.setup.Server.URL+"/api/dashboard/preferences", bytes.NewBuffer(body))
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("Authorization", "Bearer "+token)
+
+	client := &http.Client{}
+	resp, err := client.Do(req)
+	require.NoError(s.T(), err)
+	defer func(Body io.ReadCloser) {
+		_ = Body.Close()
+	}(resp.Body)
+
+	assert.Equal(s.T(), http.StatusBadRequest, resp.StatusCode)
+}
+
+func (s *DashboardIntegrationTestSuite) TestGetPreferences_GlobalAndPerLibraryIndependent() {
+	token := s.setup.Token
+	libraryID := createTestLibraryWithFolder(s.T(), s.setup.Server, token, "Prefs Scope Library", false)
+
+	// Global scope: 15.
+	globalBody, _ := json.Marshal(map[string]interface{}{
+		"hidden_collections": []string{},
+		"collection_order":   []string{},
+		"items_per_section":  15,
+	})
+	req, _ := http.NewRequest("PUT", s.setup.Server.URL+"/api/dashboard/preferences", bytes.NewBuffer(globalBody))
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("Authorization", "Bearer "+token)
+	client := &http.Client{}
+	resp, err := client.Do(req)
+	require.NoError(s.T(), err)
+	resp.Body.Close()
+	require.Equal(s.T(), http.StatusOK, resp.StatusCode)
+
+	// Per-library scope: 30. Must not bleed into global or vice versa.
+	libBody, _ := json.Marshal(map[string]interface{}{
+		"library_id":         libraryID,
+		"hidden_collections": []string{},
+		"collection_order":   []string{},
+		"items_per_section":  30,
+	})
+	req, _ = http.NewRequest("PUT", s.setup.Server.URL+"/api/dashboard/preferences", bytes.NewBuffer(libBody))
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("Authorization", "Bearer "+token)
+	resp, err = client.Do(req)
+	require.NoError(s.T(), err)
+	resp.Body.Close()
+	require.Equal(s.T(), http.StatusOK, resp.StatusCode)
+
+	// Per-library read.
+	req, _ = http.NewRequest("GET", s.setup.Server.URL+"/api/dashboard/preferences?library_id="+libraryID, nil)
+	req.Header.Set("Authorization", "Bearer "+token)
+	resp, err = client.Do(req)
+	require.NoError(s.T(), err)
+	defer func(Body io.ReadCloser) {
+		_ = Body.Close()
+	}(resp.Body)
+	require.Equal(s.T(), http.StatusOK, resp.StatusCode)
+	var libPrefs map[string]interface{}
+	err = json.NewDecoder(resp.Body).Decode(&libPrefs)
+	require.NoError(s.T(), err)
+	assert.Equal(s.T(), float64(30), libPrefs["items_per_section"], "per-library scope should read its own row")
+
+	// Global read.
+	req, _ = http.NewRequest("GET", s.setup.Server.URL+"/api/dashboard/preferences", nil)
+	req.Header.Set("Authorization", "Bearer "+token)
+	resp, err = client.Do(req)
+	require.NoError(s.T(), err)
+	defer func(Body io.ReadCloser) {
+		_ = Body.Close()
+	}(resp.Body)
+	require.Equal(s.T(), http.StatusOK, resp.StatusCode)
+	var globalPrefs map[string]interface{}
+	err = json.NewDecoder(resp.Body).Decode(&globalPrefs)
+	require.NoError(s.T(), err)
+	assert.Equal(s.T(), float64(15), globalPrefs["items_per_section"], "global scope should stay independent")
+}
+
 func TestDashboardIntegrationTestSuite(t *testing.T) {
 	suite.Run(t, new(DashboardIntegrationTestSuite))
 }