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.

internal/handlers/stats.go

@@ -50,14 +50,25 @@ func (h *StatsHandler) Summary(c *echo.Context) error {
 
 // ThisWeek handles GET /api/stats/this-week — the dashboard widget's
 // data: rolling 7-day totals vs the previous seven days, plus streak.
+// An optional ?library_id scopes the week to one library (the
+// dashboard's library switcher drives it).
 func (h *StatsHandler) ThisWeek(c *echo.Context) error {
 	user, ok := mustUser(c)
 	if !ok {
 		return echo.NewHTTPError(http.StatusUnauthorized, "invalid user")
 	}
+	var libraryID pgtype.UUID
+	if raw := c.QueryParam("library_id"); raw != "" {
+		parsed, err := uuid.Parse(raw)
+		if err != nil {
+			return echo.NewHTTPError(http.StatusBadRequest, "invalid library id")
+		}
+		libraryID = pgtype.UUID{Bytes: parsed, Valid: true}
+	}
 	week, err := h.statsService.ThisWeek(c.Request().Context(),
 		pgtype.UUID{Bytes: user.ID.Bytes, Valid: true},
 		user.Timezone.String,
+		libraryID,
 	)
 	if err != nil {
 		return echo.NewHTTPError(http.StatusInternalServerError, err.Error())