feat(dashboard): collapsible This Week widget, library-scoped + switch-surviving; title into the subbar

Three dashboard complaints, one build:

- The widget collapses to a single summary row (This Week · minutes ·
  pages · sessions · books · streak · Full stats) on every screen size
  and expands to the full grid via a details/summary toggle — the
  carousel sections start immediately below it again.
- The 'Your Library' heading moved into the library switcher subbar
  (LibrarySwitcher gains an optional title param; collections/series
  keep the plain 'Library' label). The standalone heading block is
  gone.
- Root-caused the JS-switch vanishing: renderDashboardCollections
  replaces ALL of #collections-container's innerHTML, so anything
  server-rendered inside it (title, widget) died on the first library
  switch. The widget now re-fetches ITSELF: a /dashboard/week-widget
  fragment (same resolveLibrary family-lens rules as the page) is
  pulled by htmx whenever the switcher dispatches week-refresh —
  carrying the current #library-select value — so the widget both
  survives switches and RE-SCOPES to the switched library. Initial SSR
  renders the same scope the sections use.

ThisWeek (and the streak calendar) gained an optional library filter.
Tests: TestReadingStatsWeekLibraryScope — per-library totals, scoped
streaks, invisible-library isolation, malformed id 400. Full suite
PASS.

templates/series.templ

@@ -17,7 +17,7 @@ templ Series(user User, seriesList []SeriesCardData, libData []LibraryData, curr
 		</head>
 		<body x-data="seriesPage" x-init="initSeriesPage()" class="theme-{ user.Theme }">
 			@Header(user, "/series")
-			@LibrarySwitcher(libData, currentLibraryID)
+			@LibrarySwitcher(libData, currentLibraryID, "")
 			<div id="series-container">
 				<main class="mx-auto container px-4 sm:px-6 lg:px-8 py-8">
 					<div class="mb-6 flex items-center gap-3">