Guard key material in RAM with memguard, add Markdown preview for message field

Reader now keeps the private key bytes and the per-submission ECDH/HKDF
secrets in memguard.LockedBuffer (mlock + guaranteed wipe) instead of a
manual zero loop. Added a regression test for the decrypt path.

Message field gets a self-hosted Markdown toggle preview (no external
library): headers, bold/italic, inline code, code blocks, blockquotes,
lists, links. Output is HTML-escaped and link schemes are whitelisted
before rendering. Submitted content is still the raw Markdown source,
unchanged on the wire.

1 files changed, 7 insertions, 0 deletions

diff --git a/static/crypto.js b/static/crypto.js
index f1cc6c9..e196826 100644
--- a/static/crypto.js
+++ b/static/crypto.js
@@ -161,6 +161,13 @@ document.getElementById("drop-form").addEventListener("submit", async (e) => {
         if (resp.ok) {
             document.getElementById("drop-form").reset();
             document.getElementById("filename").textContent = "";
+            const preview = document.getElementById("message-preview");
+            const previewToggle = document.getElementById("preview-toggle");
+            if (preview && previewToggle && !preview.hidden) {
+                preview.hidden = true;
+                document.getElementById("message").hidden = false;
+                previewToggle.textContent = "Preview";
+            }
             status.textContent = "Delivered. No record of this submission exists on the server.";
         } else {
             status.className = "error";