fix(security): apply audit remediations to dev (ported + dev-only)

Dev-branch audit (docs/_archive/security-audit-dev-2026-07-22.md) re-found the
master issues (absent on dev) plus new ones in dev-only code. Applied here;
full-node build + test suite green; the subtle fixes were adversarially re-verified.

Ported from the master remediation (adapted to dev's code):
- bootstrap: reject zip-slip / path-traversal archive members (isSafeArchivePath)
  before writing (S2-1). (dev already fail-closes on a missing checksum.)
- xmrig updater: fail closed when a signature is required but no key is pinned (F1-1).
- http_download.httpGetString: 16 MiB hard cap + MAXFILESIZE on the shared
  metadata/price fetch (F1-2 / caps the updater + exchange paths at one site).
- rpc_client: explicit SSL_VERIFYPEER/VERIFYHOST (F4-1) and a 256 MiB response
  cap in WriteCallback (F4-3).
- lite_connection_service: reject remote http:// lite servers, loopback only (L1-1).
  Loopback is matched by a strict dotted-decimal 127.0.0.0/8 check (not a
  startsWith("127.") prefix, which would wrongly accept 127.0.0.1.evil.com), with
  userinfo/fragment stripping.
- lite controller: propagate encrypt/decrypt save() failure instead of reporting
  success (F7-1).
- xmrig_manager: chmod(0600) the pool config before writing secrets (F5-2).
- app: clear the copied secret from the OS clipboard on shutdown (F3b-1).
- export_transactions: neutralize CSV/spreadsheet formula injection (F13-1).
- build pipeline: build-from-source lite backend + remove the self-attested
  CMake signature gate (F15-1); pinned+verified appimagetool (F15-3/4);
  verified Sapling params in setup.sh (F15-6); build.sh exits 0 on success.
  (F14-1 empty-quoted-arg and F8-2 NUL-termination were already fixed on dev.)

Dev-only findings:
- rpc_client.callRaw: scrub the raw buffer + parsed tree (templated scrubJsonSecrets
  for ordered_json) so console dumpprivkey/z_exportkey keys don't linger in freed
  heap (N1-1).
- seed_wallet_creator: wipe the exported mnemonic on the failure path so a discarded
  failed result never carries a live seed (W1-2).
- export_all_keys: write the plaintext key dump 0600 + atomically via
  writeFileAtomically (U1-2).
- chat_database: restrict chat_messages.sqlite and its WAL/SHM sidecars to owner-only (C3-1).

Not done (need a decision, documented in the report):
- Chat header metadata (cid/z/p) rides outside the AEAD (C1/C2) — binding it is a
  wire-protocol change requiring SilentDragonXLite interop review.
- Bootstrap lacks an offline-rooted signature (S2-2 residual) — needs signing infra.
- Console scrollback retains console-typed key-export output in plaintext (N1-1
  residual) — inherent to an echoing console; would need output redaction.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

build.sh

@@ -478,18 +478,28 @@ APPRUN
     done
     [[ -f "$bd/_deps/sdl3-build/libSDL3.so" ]] && cp "$bd/_deps/sdl3-build/libSDL3.so"* "$APPDIR/usr/lib/" 2>/dev/null || true
 
-    # appimagetool
+    # appimagetool — pinned to a tagged release and SHA-256 verified before we exec it.
+    # The old "continuous" tag is a MOVING build fetched over the network and run on the release
+    # builder; a compromised/MITM'd artifact would execute here. Verify, or refuse to package.
+    local APPIMAGETOOL_URL="https://github.com/AppImage/appimagetool/releases/download/1.9.0/appimagetool-x86_64.AppImage"
+    local APPIMAGETOOL_SHA256="46fdd785094c7f6e545b61afcfb0f3d98d8eab243f644b4b17698c01d06083d1"
     local APPIMAGETOOL=""
     if command -v appimagetool &>/dev/null; then
-        APPIMAGETOOL="appimagetool"
-    elif [[ -f "$bd/appimagetool-x86_64.AppImage" ]]; then
-        APPIMAGETOOL="$bd/appimagetool-x86_64.AppImage"
+        APPIMAGETOOL="appimagetool"   # maintainer's own trusted system install
     else
-        info "Downloading appimagetool ..."
-        wget -q -O "$bd/appimagetool-x86_64.AppImage" \
-            "https://github.com/AppImage/AppImageKit/releases/download/continuous/appimagetool-x86_64.AppImage"
-        chmod +x "$bd/appimagetool-x86_64.AppImage"
-        APPIMAGETOOL="$bd/appimagetool-x86_64.AppImage"
+        local at="$bd/appimagetool-x86_64.AppImage"
+        # Re-verify any cached copy too; a stale unverified download must not be trusted.
+        if [[ ! -f "$at" ]] || ! echo "${APPIMAGETOOL_SHA256}  ${at}" | sha256sum -c --status; then
+            info "Downloading appimagetool 1.9.0 (pinned) ..."
+            wget -q -O "$at" "$APPIMAGETOOL_URL"
+            if ! echo "${APPIMAGETOOL_SHA256}  ${at}" | sha256sum -c --status; then
+                err "appimagetool SHA-256 verification failed — refusing to use it"
+                rm -f "$at"
+                return 1
+            fi
+            chmod +x "$at"
+        fi
+        APPIMAGETOOL="$at"
     fi
 
     local ARCH
@@ -1311,3 +1321,9 @@ if $DO_LINUX || $DO_WIN || $DO_MAC; then
     [[ -d "$SCRIPT_DIR/release/windows" ]] && echo -e "    ${CYAN}windows/${NC}   — .exe + .zip"
     [[ -d "$SCRIPT_DIR/release/mac" ]]     && echo -e "    ${CYAN}mac/${NC}       — .app + .dmg"
 fi
+
+# Reaching here means the build completed (real failures exit 1 at their point of failure).
+# Exit 0 explicitly: the final `[[ -d release/mac ]] && echo` above returns non-zero on a
+# non-mac build — and since set -e exempts the left side of an &&, that status would otherwise
+# become the script's exit code and make a successful build report failure (e.g. to CI).
+exit 0