v4.0.22 — Kobo cover padding default-on + UI polish

Suggested release title:

v4.0.22 — pad covers by default with Kobo sync + Save/Cancel polish

Release body

A polish release on top of v4.0.21. Three operator-driven UX fixes:

docker pull ghcr.io/new-usemame/calibre-web-nextgen:v4.0.22

Same volume layout as v4.0.21; no migration.

Cover padding now defaults to ON when Kobo sync is enabled. When you (or any user) first checks "Enable Kobo sync" and saves, the new cover-padding section auto-enables in the same save. You can disable padding independently after — the auto-enable only fires on the off→on transition of Kobo sync, so a user choice to turn padding off is preserved on subsequent saves. New installs also get padding enabled in the column default.
The "Custom border color" input is now locked outside manual fill mode. When you pick Edge mirror, Edge blur, Average, or Dominant as the fill style, the color input dims to 50% opacity and goes read-only — clearly signaling it isn't being used, but preserving the typed value so switching back to manual restores your color. Layout doesn't shift.
Save / Cancel buttons reordered + Cancel restyled. Across all settings pages, the Save/Cancel pair is now both right-aligned with Save on the rightmost edge, Cancel to its left. Cancel uses a new secondary style — white background, brand-orange text + matching border — so primary and secondary actions are visually distinct. Touched: Configuration, DB Configuration, View Configuration, Schedule, Shelf editor, Book metadata editor, User editor.

End-to-end browser-driven against the deployed instance under caliBlur dark theme:

Save text right of Cancel text (positions verified)
Cancel computed style: white background, orange #cc7b19 text + border
cwa-form-actions flex-end container correctly applied
Fill mode change to manual → color input becomes editable, opacity 1
Fill mode change to anything else → color input read-only, opacity 0.5
Round-trip: typed manual color preserved across mode toggles
27 unit tests + 17 Wand-gated tests still green; live boot on teenyverse healthy on first restart with new image