feat: persist pinned libraries per user with cross-device sync (#20)

Store pinned library IDs on the user record and expose GET/PUT
/me/pinned-libraries endpoints. The web client now loads and saves pins
through the API, migrates legacy localStorage data once, and applies
pinned ordering on both the libraries and home pages.

Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: truewhile <truewhile@users.noreply.github.com>

internal/handler/routes_authenticated_test.go

@@ -25,6 +25,8 @@ func TestAuthenticatedRouteSurfacesAreRegistered(t *testing.T) {
 
 	for _, want := range []string{
 		"GET /api/me",
+		"GET /api/me/pinned-libraries",
+		"PUT /api/me/pinned-libraries",
 		"GET /api/auth/permissions",
 		"GET /api/libraries",
 		"GET /api/media",