refactor(handlers): update all handlers for Echo v5 compatibility

Update all handler functions to use *echo.Context (pointer) instead of echo.Context (value) as required by Echo v5.

Changes across all handler files:
- analytics.go: Update handler signatures
- auth.go: Update authentication handler signatures
- book_matching.go: Update matching handler signatures
- collections.go: Update collection handler signatures
- collections_preview_test.go: Update test signatures
- commonhandlers.go: Update common handler signatures
- conflicts.go: Update conflict handler signatures
- context.go: Update context handler signatures
- dashboard.go: Update dashboard handler signatures
- devices.go: Update device handler signatures
- jobs.go: Update job handler signatures
- kobo.go: Update Kobo handler signatures
- koreader.go: Update Koreader handler signatures
- library.go: Update library handler signatures
- matching.go: Update matching handler signatures
- media.go: Update media handler signatures
- opds.go: Update OPDS handler signatures
- progress.go: Update progress handler signatures
- queue.go: Update queue handler signatures
- refresh_token.go: Update token handler signatures
- scanner.go: Update scanner handler signatures
- sidecar.go: Update sidecar handler signatures
- sync.go: Update sync handler signatures
- system_settings.go: Update settings handler signatures
- websocket.go: Update WebSocket handler signatures

All handlers now properly implement Echo v5's pointer-based context pattern.
This change is necessary for type safety and compatibility with Echo v5's
improved context handling and WebSocket support.

internal/handlers/sync.go

@@ -7,7 +7,7 @@ import (
 
 	"github.com/google/uuid"
 	"github.com/jackc/pgx/v5/pgtype"
-	"github.com/labstack/echo/v4"
+	"github.com/labstack/echo/v5"
 )
 
 type SyncHandler struct {
@@ -52,7 +52,7 @@ type LinkBookResponse struct {
 
 // GetUnlinkedBooks returns all unlinked books for a user's devices
 // GET /api/sync/unlinked-books
-func (h *SyncHandler) GetUnlinkedBooks(c echo.Context) error {
+func (h *SyncHandler) GetUnlinkedBooks(c *echo.Context) error {
 	// Get user from context (assuming auth middleware sets this)
 	user := c.Get("user")
 	if user == nil {
@@ -105,7 +105,7 @@ func (h *SyncHandler) GetUnlinkedBooks(c echo.Context) error {
 
 // LinkBook manually links an unlinked book to a media item
 // POST /api/sync/link-book
-func (h *SyncHandler) LinkBook(c echo.Context) error {
+func (h *SyncHandler) LinkBook(c *echo.Context) error {
 	var req LinkBookRequest
 	if err := c.Bind(&req); err != nil {
 		return c.JSON(http.StatusBadRequest, map[string]string{