The All Libraries entry in the library switcher could render the
cross-library dashboard but rejected its own settings: the modal
refused to open without a library ('Select a specific library...') and
a save sent library_id: '' (server 400). The server now stores a
global scope (separate table; null/absent library_id, empty string
still 400), so:
- openDashboardSettings allows the All view and fetches prefs without
the library param (reads the global row; previously a loose NULL
OR-clause matched an arbitrary library's row).
- saveDashboardSettings sends library_id: null from All Libraries.
- The settings modal gains a scope label (#dashboard-scope-label)
showing which library — or All Libraries — a save will customize
(user requirement: always know the scope).
Verified end-to-end in headless Chromium: reorder on All Libraries →
save (null) → reload → modal re-opens with the persisted order and the
All Libraries scope label; per-library save lands in its own row
without touching global (SQL-checked).