feat(design): the page band (B4) — one h1 per page, section headings, skip to content, filters fold on a short window (E28 S3d4)

- PageShell names the page with a visually hidden h1; a view with its own title renders it as
  PageHeading and the hidden one steps aside (contract: no hand <h1> inside the shell).
- Section captions are SectionLabel headings (h2, h3 in a dialog), ui/form Section included.
- "Skip to content" is the first Tab stop and focuses the scroller's <main>.
- Below 600px of height the band's filter row folds into "Filters · n" (Feed, Plex, server log),
  the same row shown in place.
- The band holds controls only: Downloads and Notifications lose their title cards, the Config and
  channel-discovery intros scroll with the content, the Stats tabs and the audit pager join the band,
  and the server log's toolbar sticks with the fade under it.

frontend/src/components/ui/README.md

@@ -44,7 +44,7 @@ domain-free.
 B5 glass roles; a bare glass class is a lint error), `Badge` (a static label), `LinkButton` / `IconLink` (a
 link wearing the button face) over `buttonStyles`, `Skeleton` (B8 first loads, through `LoadingState shape`), `NavRail` (B7), `form`, `DraftSaveBar`, `Combobox`, `FilterSelect`, `SegmentedToggle`, `SortControl`, `ColumnSortControl`, `PriorityStepper`, `UndoToolbar` |
 | data display | `DataTable`, `tableHeader` + `tableAlign`, `Pager` + `pagerContent`, `VirtualGrid` + `gridColumns` (the column/width maths, incl. the ultrawide `maxCols` cap), `readAhead`, `Carousel`, `NavGrid` (a plain grid, list or table body the keyboard walks — `DataTable`, `VirtualGrid` and `Carousel` take `keyboardNav`), `Avatar`, `CountBadge`, `ItemCount`, `FilterChipGroup` |
-| page & panel structure | `PageShell` + `pageToolbarSlot`, `PageScroller`, `PanelGroup`, `PanelGroups`, `Tabs`, `BackToTop` |
+| page & panel structure | `PageShell` + `pageToolbarSlot` + `pageHeading`, `PageScroller`, `Headings` (`PageHeading`, `SectionLabel`), `SkipLink` + `landmarks`, `FilterFold` (B4), `PanelGroup`, `PanelGroups`, `Tabs`, `BackToTop` |
 | state & messaging | `QueryState`, `Banner`, `InlineNotice` |
 
 ## The size contract (E10 S1c)