feat(router): cookie-aware SSR library resolution with resolveLibrary helper

- helpers.go: Promote getText() from a local closure in frontend.go
  to a package-level function so it can be used by resolveLibrary.
  Add resolveLibrary(c, cfg, user.ID) helper that:
    1. Reads library_id query param (explicit navigation wins)
    2. Falls back to selectedLibrary cookie — validates __all__
       sentinel or real UUID, rejects garbage values silently
    3. Falls back to user's first visible library
  Returns LibraryResolution struct with LibraryID, IsAll, LibUUID,
  Libraries, and FirstID — eliminating repeated boilerplate across
  all SSR routes.

- frontend.go: Replace manual library resolution boilerplate in 5
  SSR route handlers (series, tags/detail, bookshelf, dashboard,
  collections/:id) with resolveLibrary(). Each route now gets cookie-
  aware library selection for free. Collection detail correctly
  handles All Libraries mode for both system and user collections.
  Dashboard no longer makes a redundant second GetUserVisibleLibraries
  call.

internal/router/helpers.go

@@ -3,7 +3,9 @@ package router
 import (
 	"context"
 	"log"
+	"net/url"
 
+	"bookhoard/internal/database"
 	"bookhoard/templates"
 
 	"github.com/google/uuid"
@@ -86,3 +88,77 @@ func parseUUID(s string) (uuid.UUID, error) {
 func uuidToPGType(u uuid.UUID) pgtype.UUID {
 	return pgtype.UUID{Bytes: u, Valid: true}
 }
+
+const selectedLibraryCookie = "selectedLibrary"
+const allLibrariesSentinel = "__all__"
+
+type LibraryResolution struct {
+	LibraryID  string
+	IsAll      bool
+	LibUUID    pgtype.UUID
+	Libraries  []templates.LibraryData
+	FirstID    string
+}
+
+func getText(t pgtype.Text) string {
+	if t.Valid {
+		return t.String
+	}
+	return ""
+}
+
+func resolveLibrary(c *echo.Context, cfg *Config, userUUID string) LibraryResolution {
+	res := LibraryResolution{}
+
+	userU, _ := uuid.Parse(userUUID)
+	libraries, err := cfg.Queries.GetUserVisibleLibraries(c.Request().Context(), uuidToPGType(userU))
+	if err != nil {
+		log.Printf("GetUserVisibleLibraries failed: %v", err)
+		libraries = []database.GetUserVisibleLibrariesRow{}
+	}
+
+	res.Libraries = make([]templates.LibraryData, len(libraries))
+	for i, lib := range libraries {
+		libUUID, _ := uuid.FromBytes(lib.ID.Bytes[0:16])
+		res.Libraries[i] = templates.LibraryData{
+			ID:          libUUID.String(),
+			Name:        lib.Name,
+			Description: getText(lib.Description),
+			TypeName:    lib.TypeName,
+		}
+	}
+
+	if len(libraries) > 0 {
+		libUUID, _ := uuid.FromBytes(libraries[0].ID.Bytes[0:16])
+		res.FirstID = libUUID.String()
+	}
+
+	libraryID := c.QueryParam("library_id")
+	if libraryID == "" {
+		if cookie, err := c.Cookie(selectedLibraryCookie); err == nil {
+			val, _ := url.QueryUnescape(cookie.Value)
+			if val == allLibrariesSentinel {
+				res.IsAll = true
+				res.LibraryID = ""
+				return res
+			}
+			if _, parseErr := uuid.Parse(val); parseErr == nil {
+				libraryID = val
+			}
+		}
+	}
+
+	if libraryID == "" {
+		res.LibraryID = res.FirstID
+		if res.LibraryID != "" {
+			parsed, _ := uuid.Parse(res.LibraryID)
+			res.LibUUID = pgtype.UUID{Bytes: parsed, Valid: true}
+		}
+		return res
+	}
+
+	res.LibraryID = libraryID
+	parsed, _ := uuid.Parse(libraryID)
+	res.LibUUID = pgtype.UUID{Bytes: parsed, Valid: true}
+	return res
+}