refactor: one PNG metadata parser

admin.js, gallery.js and metadata.html each carried a copy of the PNG chunk
walker that reads ComfyUI's tEXt/iTXt entries (admin.js had two until the
shadowed one was removed). web/js/png-metadata.js is now the only parser:
it checks the signature, bounds-checks every chunk so a truncated file or a
bogus length cannot throw, decodes tEXt as Latin-1 and iTXt as UTF-8, skips
compressed zTXt/iTXt payloads instead of decoding them as garbage, and stops
at IEND. The three page-level parsePNGMetadata functions delegate to it.

web/admin.html

@@ -7,6 +7,7 @@
     <script src="/prompt_manager/js/theme-bridge.js"></script>
     <script src="/prompt_manager/js/html-escape.js"></script>
     <script src="/prompt_manager/js/api-client.js"></script>
+    <script src="/prompt_manager/js/png-metadata.js"></script>
     <script src="/prompt_manager/js/list-state.js"></script>
     <script src="/prompt_manager/js/sse-stream.js"></script>
     <script src="/prompt_manager/js/data-actions.js"></script>