Implement wisp: async encrypted single-retrieval file drop

Client-side libsodium encryption with the key in the URL fragment, an
Express/SQLite server holding ciphertext until a confirm-token round trip
proves successful decrypt (avoiding the delete-on-first-byte race), TTL
sweep for unclaimed drops, and a password-gated upload UI styled to match
flit. Dockerized to match the project family's conventions, with a named
volume so the DB/blobs survive redeploys, and a Gitea Actions workflow to
build and push the image.

client/public/favicon.svg

@@ -0,0 +1,6 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 32 32">
+  <rect width="32" height="32" rx="7" fill="#080808"/>
+  <text x="16" y="23" text-anchor="middle"
+        font-family="'JetBrains Mono','Fira Code',ui-monospace,monospace"
+        font-size="19" font-weight="700" fill="#00e87a">&gt;_</text>
+</svg>