feat(web): Interface A — Encode/Decode tabs + .pbf.json decode & metadata (issue #1)

Rebuilds the browser demo around explicit Encode/Decode tabs (Peter's pick) so
decoding is discoverable — the issue's core complaint. Decode accepts a pasted
block OR a dropped file, auto-routes container vs raw (decodeText), shows a
metadata panel (filename/size/modified/mode/integrity), and downloads the
restored file under its original name. Encode gains a .pbf.json container output
option (embeds filename + modified_ms + crc32).

Adds decodeText (decode-mode router, robust to raw output starting with '{') and
serializes container 'data' LAST so metadata sits up front (Peter's ask). JS
suite now 54 tests (routing + corrupt-container rejection + data-last order).

Author: pmarreck

docs/plans/2026-06-26-printable-binary-file-container-design.md

@@ -42,18 +42,24 @@ NEW option, not a replacement.
   "format": "printable-binary-file",
   "version": 1,
   "filename": "photo.png",
-  "data": "<printable-binary-encoded string>",
   "byte_length": 12345,
   "crc32": "cbf43926",
   "crc32_encoded": "1a2b3c4d",
   "modified_ms": 1719430000000,
   "created_ms": 1719420000000,
   "mode": "0644",
   "owner": "pmarreck",
-  "group": "staff"
+  "group": "staff",
+  "data": "<printable-binary-encoded string>"
 }
 ```
 
+Serialized with `data` LAST so all metadata sits up front (JS object key order
+is insertion order). `created_ms` is birthtime — populated by the CLI (which can
+`stat` it); browsers expose only `lastModified` (-> `modified_ms`), so the web
+encoder omits `created_ms` (the web decoder still shows it when a CLI-made
+container carries it).
+
 ### Field semantics
 | key            | type    | required | meaning |
 |----------------|---------|----------|---------|