feat(dashboard): move This Week stats into the library bar as a popover

The "This Week" reading-stats strip leaves its full-width row below the
subbar and now lives inside the library bar itself, between the switcher
and the customize/refresh actions. Expanding it opens an absolutely
positioned popover that FLOATS over the collections — opening the stats
no longer shifts the page down.

- LibrarySwitcher gains an optional `center` component slot rendered
  between the left group and the actions group (nil = unused). Series
  and collections call sites pass nil and render byte-identical output.
- DashboardWeekWidget redesigned for the slot: compact inline summary
  (icon, "This Week", minutes; pages/sessions/books/streak reveal at
  md/lg/xl) and a card popover panel with the labeled grid, sync note,
  and the Full stats link moved in from the old summary row.
- Panel anchoring is responsive: below `sm` it anchors to the sticky
  bar itself (details goes static; bar-anchored left-4/right-4, i.e.
  screen-centered); at `sm`+ the details is relative again and the
  panel centers on the widget (w-[30rem], translate-x centering).
- On phones the summary collapses to the chart icon + chevron (bar row
  width is scarce); "This Week"/minutes reveal at `sm`, and the
  "Your Library" title hides below `sm` to keep the row from
  overlapping. Icon-only toggle carries an aria-label.
- web/src/dashboard.ts: click-outside and Escape close the popover,
  re-querying #week-widget per event because htmx replaces it on
  library switches. Rendering untouched (behavior-only island).
- id="week-widget" and all htmx wiring unchanged: week-refresh refetch
  + the /dashboard/week-widget fragment still drive library-scoped
  refreshes behind the switch fade; stats-off suppression now yields an
  empty slot via weekWidgetComponent instead of a hidden strip.
- web/static/style.css rebuilt for the new utilities.

Known behavior (unchanged or deliberate): the bar is sticky, so stats
stay visible while scrolling; a library switch re-renders the details
closed; scopes with no current-week reading show zeros (e.g. Manga).

templates/collections.templ

@@ -218,7 +218,7 @@ templ CollectionDetail(user User, collection CollectionData, books []handlers.Bo
 		</head>
 		<body x-data="collections" x-init="initCollectionsPage()" class={ "theme-" + user.Theme }>
 			@Header(user, "/collections")
-			@LibrarySwitcher(libData, libraryID, "")
+			@LibrarySwitcher(libData, libraryID, "", nil)
 			<div class="mx-auto container px-4 sm:px-6 lg:px-8 py-8">
 				<div class="mb-6">
 					<a href="/collections" class="btn btn-secondary mb-4">