fix: wipe long-lived credential copies from memory

Session-only credentials (kept for a whole session so reconnects don't
re-prompt), credentials pending until a connection succeeds, and the
credential prompt's text buffer are now Zeroizing and wiped when
dropped, including when a prompt is cancelled or abandoned
(SEC-20261012-007, partial).

Still open: the values handed to the protocol clients are plain Strings
for the duration of authentication, egui keeps its own copies of typed
text, and no mlock/PR_SET_DUMPABLE is applied.

rust/app/Cargo.toml

@@ -13,6 +13,7 @@ nexus-core = { path = "../core" }
 eframe = { version = "0.36", default-features = false, features = ["default_fonts", "glow", "wayland", "x11", "accesskit", "persistence"] }
 egui = "0.36"
 parking_lot = "0.12"
+zeroize = "1"
 egui-phosphor = "0.14"
 rfd = "0.17"
 image = { version = "0.25", default-features = false, features = ["png"] }