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/router/frontend.go

@@ -473,13 +473,18 @@ func registerFrontendRoutes(cfg *Config) {
 		sectionData := handlers.MarkActiveConflictsSections(c.Request().Context(), cfg.Queries, userPgID, handlers.BuildSections(visibleSections, libraryID))
 		allSectionsData := handlers.BuildSections(allSections, libraryID)
 
-		// “This week” widget: the user's rolling 7-day reading pulse, from
-		// the same session seed the /stats page uses. Suppressed when the
-		// service fails (dashboard always renders).
+		// "This week" widget: the user's rolling 7-day reading pulse for
+		// the CURRENTLY SELECTED library (the same resolution the sections
+		// use), from the same session seed the /stats page serves.
+		// Suppressed when the service fails (dashboard always renders).
 		var week *templates.WeekStatsData
 		if cfg.StatisticsService != nil {
+			weekScope := pgtype.UUID{}
+			if !libRes.IsAll {
+				weekScope = libRes.LibUUID
+			}
 			weekSvc, weekErr := cfg.StatisticsService.ThisWeek(c.Request().Context(),
-				pgtype.UUID{Bytes: userUUID, Valid: true}, user.Timezone)
+				pgtype.UUID{Bytes: userUUID, Valid: true}, user.Timezone, weekScope)
 			if weekErr == nil {
 				week = &templates.WeekStatsData{
 					WeekMinutes:   weekSvc.WeekMinutes,
@@ -489,6 +494,7 @@ func registerFrontendRoutes(cfg *Config) {
 					PrevMinutes:   weekSvc.PrevMinutes,
 					ActiveToday:   weekSvc.ActiveToday,
 					CurrentStreak: weekSvc.CurrentStreak,
+					LibraryID:     libRes.LibraryID,
 				}
 			} else {
 				log.Printf("ThisWeek widget failed: %v", weekErr)
@@ -840,6 +846,46 @@ func registerFrontendRoutes(cfg *Config) {
 		return c.HTML(http.StatusOK, buf.String())
 	})
 
+	// Week-widget fragment: the collapsible "This Week" strip re-fetches
+	// itself here (htmx) whenever the library switcher dispatches
+	// week-refresh, carrying the current #library-select value. Same
+	// resolution rules as the dashboard page (family-lens safe).
+	frontendProtected.GET("/dashboard/week-widget", func(c *echo.Context) error {
+		user, err := getTemplateUserWithTheme(c, cfg)
+		if err != nil {
+			return echo.NewHTTPError(http.StatusUnauthorized, "invalid user")
+		}
+		if cfg.StatisticsService == nil {
+			return echo.NewHTTPError(http.StatusServiceUnavailable, "stats unavailable")
+		}
+		libRes := resolveLibrary(c, cfg, user.ID)
+		weekScope := pgtype.UUID{}
+		if !libRes.IsAll {
+			weekScope = libRes.LibUUID
+		}
+		userUUID, _ := uuid.Parse(user.ID)
+		weekSvc, weekErr := cfg.StatisticsService.ThisWeek(c.Request().Context(),
+			uuidToPGType(userUUID), user.Timezone, weekScope)
+		if weekErr != nil {
+			return echo.NewHTTPError(http.StatusInternalServerError, weekErr.Error())
+		}
+		var buf bytes.Buffer
+		err = templates.DashboardWeekWidget(templates.WeekStatsData{
+			WeekMinutes:   weekSvc.WeekMinutes,
+			WeekPages:     weekSvc.WeekPages,
+			WeekSessions:  weekSvc.WeekSessions,
+			WeekBooks:     weekSvc.WeekBooks,
+			PrevMinutes:   weekSvc.PrevMinutes,
+			ActiveToday:   weekSvc.ActiveToday,
+			CurrentStreak: weekSvc.CurrentStreak,
+			LibraryID:     libRes.LibraryID,
+		}).Render(c.Request().Context(), &buf)
+		if err != nil {
+			return err
+		}
+		return c.HTML(http.StatusOK, buf.String())
+	})
+
 	// Custom Section Builder page
 	frontendProtected.GET("/custom-section", func(c *echo.Context) error {
 		user, err := getTemplateUserWithTheme(c, cfg)