feat(reader): webtoon reading mode + brightness/contrast/night filters

Phase 4 of the reader redesign (foliate-js ea268df):

- Webtoon mode for comics: continuous vertical scroll of all pages
  (900px centered column on wide screens), lazy-loaded with a 150%
  IntersectionObserver margin, far pages unloaded to bound memory
  with stable aspect-ratio placeholders so the scrollbar never jumps.
  Chosen per book (Paged | Webtoon segmented control in Settings →
  Layout & Display; stored in localStorage per media item since a
  webtoon title and a paged manga volume want different flows).
  Toggling reloads the reader — the renderer is chosen at open time —
  and progress restores from the saved page. Relocate events flow
  through the same pipeline, so the slider, progress saving, back
  stack, tap zones, and edge zones all work unchanged. Zoom/fit/
  magnifier/spread controls hide in webtoon (natural-width scroll).
- Display filters for fixed-layout: brightness (30-130%) and
  contrast (70-130%) sliders with live preview, plus Night Mode
  (invert) — also a quick row in the ⋯ tools menu. One --fx-filter
  CSS var drives everything: ::part(filter) on foliate-view iframes
  (forwarded via the new exportparts attribute) and the webtoon
  page images alike. Persisted as fx_brightness/fx_contrast/fx_invert
  (types + defaults both sides); Restore Defaults resets them.

internal/services/reader.go

@@ -405,6 +405,9 @@ func (s *ReaderService) getDefaultSettings() map[string]interface{} {
 		"panel_zoom_enabled":   true,
 		"double_page_spread":   true,
 		"pdf_interaction_mode": "select",
+		"fx_brightness":        1,
+		"fx_contrast":          1,
+		"fx_invert":            false,
 		"tap_zones_enabled":    true,
 
 		// Dockable panel defaults