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.

web/src/dashboard.ts

@@ -399,9 +399,12 @@ function initDashboard() {
 
   initLibrarySwitcher({
     onSwitch: (libraryId) =>
-      switchWithTransition("collections-container", () =>
-        fetchAndRenderSections(libraryId),
-      ),
+      switchWithTransition("collections-container", async () => {
+        await fetchAndRenderSections(libraryId);
+        // The "This Week" widget tracks the switched library: htmx
+        // re-fetches its fragment with the current #library-select value.
+        document.body.dispatchEvent(new CustomEvent("week-refresh"));
+      }),
   });
 
   document.addEventListener("click", (e: Event) => {