v0.5.4: code-cleanup pass (AppError, figment config, encoding dedup, typed status, deps)

Final cleanup before hardware testing. No behaviour changes; 248 tests green,
clippy clean.

#1  AppError newtype (http-api/src/error.rs) with one IntoResponse mapping
    (NotFound→404, Invalid→400, _→500) + From<core::Error>/From<io::Error>.
    Converted the clearly-safe handlers (sso_put, unattended_upload,
    branding_clear) to `?`; intentionally left handlers with bespoke
    status semantics (Invalid→404 on category, 409 on duplicate share /
    open upload) explicit so no asserted status changes.
#2  figment-based Config::load (defaults → TOML → env). Keeps the historical
    flat OPENPXE_* names (Unraid/entrypoint compatible) AND adds the nested
    OPENPXE_SECTION__FIELD form; now covers every field (apply_env had
    silently skipped unattended_dir + bind addrs). 6 Jail tests prove
    backward-compat. Removed the hand-rolled apply_env.
#3  thiserror 1→2; dropped unused mime/mime_guess/once_cell deps.
#4  Re-evaluated: Duration::from_hours/from_mins are stable on the pinned
    1.95 toolchain and clippy prefers them — kept the readable form
    (the "unstable" premise didn't hold; MSRV is intentionally 1.95).
#5  insta snapshot of the rendered iPXE menu (version-filtered) + wiremock
    coverage of the SAML metadata-URL fetch (200 + non-2xx).
#6  api_status → typed StatusResponse struct (was a 25-key json! blob) with
    a full_flow guard test asserting every UI key + the started_at string
    shape. Deferred the /api/docs typed conversion (lowest value, highest
    churn, zero functional benefit).
#7  pct_encode/xml_escape de-duplicated into openpxe_core::encoding (were
    copied across app.rs + the SAML modules). No new crates.
#8  UploadSessions registry → parking_lot::RwLock (sync, never held across
    .await); per-session lock stays tokio::Mutex.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

crates/http-api/src/ipxe_script.rs

@@ -693,6 +693,22 @@ mod password_tests {
         assert!(s.contains("arm64 UEFI"), "{s}");
     }
 
+    // v0.5.4: a full snapshot of the rendered top menu. The fragment
+    // `assert!`s above check specific invariants; this catches *any* other
+    // drift (a reordered item, a dropped line, changed spacing) so it's
+    // reviewed deliberately. The OpenPXE version is filtered out so the
+    // snapshot doesn't churn on every release bump.
+    #[test]
+    fn render_menu_snapshot() {
+        // Normalize the compile-time version so the snapshot doesn't churn
+        // on every release bump (no insta `filters` feature needed).
+        let rendered = render_menu(&[], &Settings::default(), "http://10.0.0.5").replace(
+            concat!("OpenPXE v", env!("CARGO_PKG_VERSION")),
+            "OpenPXE vX.Y.Z",
+        );
+        insta::assert_snapshot!(rendered);
+    }
+
     #[test]
     fn generated_scripts_do_not_emit_bare_or_trailing_fallbacks() {
         let settings = Settings::default();