Accounts and sign-in, and fix the read toggle

epAction's redraw closure called itself when handed a row, so Mark read
recursed until the stack blew; it now swaps that row in place. Opening an
item also marks it read, redrawn where it stands so nothing vanishes from
under the pointer on the Unread tab.

Step A of multi-user: users and sessions tables, Argon2id, a session
cookie, ipx user subcommands, and a trusted proxy header for Cloudflare
Zero Trust -- honoured only from a trusted_proxies address. The shared
token still works and is the admin. A new database starts with
admin/ipodderx.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AdXho5tTkjFLeUXKbEjKBh

Cargo.toml

@@ -6,6 +6,7 @@ edition = "2024"
 [dependencies]
 ammonia = "4.1.4"
 anyhow = "1.0.104"
+argon2 = "0.6.0"
 atom_syndication = "0.12.10"
 axum = "0.8.9"
 chrono = { version = "0.4.45", default-features = false, features = ["std", "clock"] }