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>

src/ui/windows/export_transactions_dialog.cpp

@@ -36,21 +36,30 @@ static bool s_exporting = false;
 // Helper to escape CSV field
 static std::string escapeCSV(const std::string& field)
 {
-    if (field.find(',') != std::string::npos || 
-        field.find('"') != std::string::npos ||
-        field.find('\n') != std::string::npos) {
+    // Neutralize spreadsheet formula injection: a field beginning with '=', '+', '-', '@',
+    // tab, or CR is interpreted as a formula by Excel/LibreOffice, letting an attacker-supplied
+    // memo/address execute on open. Prefix such fields with a single quote so they render as text.
+    std::string safe = field;
+    if (!safe.empty()) {
+        const char c0 = safe.front();
+        if (c0 == '=' || c0 == '+' || c0 == '-' || c0 == '@' || c0 == '\t' || c0 == '\r')
+            safe.insert(safe.begin(), '\'');
+    }
+    if (safe.find(',') != std::string::npos ||
+        safe.find('"') != std::string::npos ||
+        safe.find('\n') != std::string::npos) {
         // Escape quotes and wrap in quotes
         std::string escaped;
-        escaped.reserve(field.size() + 4);
+        escaped.reserve(safe.size() + 4);
         escaped += '"';
-        for (char c : field) {
+        for (char c : safe) {
             if (c == '"') escaped += "\"\"";
             else escaped += c;
         }
         escaped += '"';
         return escaped;
     }
-    return field;
+    return safe;
 }
 
 void ExportTransactionsDialog::show()