GUI rework for source-only builds + CLI security fixes + packaging + cross-platform scaffolding

Fixes the "GUI functions don't work on GNU Guix" report and a batch of
adversarial-audit findings across the CLI, decoder, crypto and packaging.

GUI (gui/src/zupt_gui.py)
- Root cause of the broken GUI: every encryption path defaulted to the
  libzuptsdk "SDK v2" modes, which are absent from the source-only build and
  exit 1. Reworked Keys/Compress/Extract around the native modes with a
  build-aware PQ-mode selector: Hybrid (--pq, default), Full PQ (--pq-only),
  and SDK v2 only when the binary reports WITH_SDK support. Capability is
  detected from `version` ("Build:"/"KDF:" lines) with a `help` fallback.
- Extract/Verify gain a PQ private-key input with auto-detect (reads the
  archive via `info` to pick --pq vs --pq-only). Verify could not verify any
  PQ archive before (password field only).
- run_async now holds a LIST of in-flight (thread, worker) refs; DiskTab's two
  buttons previously shared one slot, so a second op GC'd the first QThread
  mid-run. About tab corrected (codec 2.60.4, PBKDF2 default, --pq-only, URL).

CLI (src/zupt_main.c)
- compress -p <archive> <files> swallowed the archive name as the password and
  truncated the first input file (silent data loss, exit 0). Added a
  data-loss guard: refuse to overwrite an existing non-.zupt file as the output
  archive unless -y/--force; plus a self-overwrite guard.
- compress <archive> <src> -p <pw> wrote an UNENCRYPTED archive (exit 0)
  because options after the first positional were treated as files. Now errors
  on a misplaced option (with a `--` escape for real dashed filenames).
- Removed duplicated dead --pq-box/--pq-sdk branches. version/banner/usage now
  state the build's real KDF (PBKDF2 on source-only) and repo URL; examples
  lead with native --pq / --pq-only.

Security (audit findings)
- vv_decoder.c: the two AVX2 fast-path token decoders read a 2-/3-byte match
  offset after only checking 1 byte remained -> heap over-read on a crafted
  archive. Added the `ip + off_bytes > ip_end` guard the tail path already had.
- zupt_crypto.c: wipe ML-KEM/X25519 secret-key buffers when hybrid decrypt
  init fails on key read (matches the pq-only path).
- zupt_format.c: bound attacker-controlled encryption_header_off before the
  (off_t)+7 arithmetic in the info enc_type reader (avoid signed-overflow UB).

Packaging (would fail source-only)
- debian/rules (staged into debian/zupt, installed vendored .so), aur/PKGBUILD,
  nix/flake.nix, homebrew (vendored .so + nonexistent AUDIT.md, stale /zupt
  URLs) now build source-only. opensuse spec %files ships the shell completions
  make install writes (rpmbuild no longer fails on unpackaged files);
  _service + debian/control point at the vaptvupt repo; KDF claim corrected.

Cross-platform GUI packaging (new)
- packaging/portable/: OS-agnostic GUI package (zupt_gui.py + .bat/.command/.sh
  launchers + README) that runs on Windows/macOS/Linux/BSD with Python+PySide6.
- .github/workflows/cross-platform.yml: builds real native binaries on
  windows-latest + macos-latest runners (CLI, PyInstaller GUI, Inno Setup
  installer, .dmg) and the portable zip, attaching them to the release on tag.
- packaging/windows/vaptvupt-gui.iss: Inno Setup installer script.

Validation: make check 16/16 (all distro-safe checks), GUI imports + command
contracts verified against the fixed CLI. test_help_consistency updated to
assert the truthful (build-aware) default KDF.

packaging/windows/vaptvupt-gui.iss

@@ -0,0 +1,73 @@
+; SPDX-License-Identifier: AGPL-3.0-or-later
+; Inno Setup script for the VaptVupt GUI Windows installer.
+;
+; Compiled by the cross-platform CI (.github/workflows/cross-platform.yml) with:
+;   ISCC.exe /DAppVersion=<version> packaging/windows/vaptvupt-gui.iss
+; after PyInstaller has produced dist\vaptvupt-gui.exe (a onefile bundle that
+; already contains Python, PySide6 and vaptvupt.exe). Requires Inno Setup 6+.
+;
+; To build locally on Windows: install Inno Setup, then run the same ISCC line
+; from the repo root (with dist\vaptvupt-gui.exe present).
+
+#ifndef AppVersion
+  #define AppVersion "0.0.0"
+#endif
+
+[Setup]
+AppName=VaptVupt
+AppVersion={#AppVersion}
+AppPublisher=Cristian Cezar Moises
+AppPublisherURL=https://git.securityops.co/cristiancmoises/vaptvupt
+DefaultDirName={autopf}\VaptVupt
+DefaultGroupName=VaptVupt
+UninstallDisplayIcon={app}\vaptvupt-gui.exe
+OutputDir=packaging\windows\Output
+OutputBaseFilename=VaptVupt-Setup-{#AppVersion}
+Compression=lzma2
+SolidCompression=yes
+ArchitecturesAllowed=x64compatible
+ArchitecturesInstallIn64BitMode=x64compatible
+WizardStyle=modern
+LicenseFile=LICENSE
+
+[Languages]
+Name: "english"; MessagesFile: "compiler:Default.isl"
+
+[Files]
+; PyInstaller onefile bundle (Python + PySide6 + the GUI + vaptvupt.exe).
+Source: "dist\vaptvupt-gui.exe"; DestDir: "{app}"; Flags: ignoreversion
+; Ship the raw CLI too so it can be added to PATH and used from a terminal.
+Source: "vaptvupt.exe"; DestDir: "{app}"; Flags: ignoreversion skipifsourcedoesntexist
+Source: "README.md"; DestDir: "{app}"; Flags: ignoreversion isreadme
+Source: "CHANGELOG.md"; DestDir: "{app}"; Flags: ignoreversion
+
+[Icons]
+Name: "{group}\VaptVupt"; Filename: "{app}\vaptvupt-gui.exe"
+Name: "{group}\Uninstall VaptVupt"; Filename: "{uninstallexe}"
+Name: "{autodesktop}\VaptVupt"; Filename: "{app}\vaptvupt-gui.exe"; Tasks: desktopicon
+
+[Tasks]
+Name: "desktopicon"; Description: "Create a desktop shortcut"; GroupDescription: "Additional icons:"
+Name: "addtopath"; Description: "Add the vaptvupt CLI to PATH (current user)"; GroupDescription: "Command line:"
+
+[Registry]
+; Optionally add the install dir to the user PATH (for the vaptvupt.exe CLI).
+Root: HKCU; Subkey: "Environment"; ValueType: expandsz; ValueName: "Path"; \
+  ValueData: "{olddata};{app}"; Tasks: addtopath; Check: NeedsAddPath('{app}')
+
+[Run]
+Filename: "{app}\vaptvupt-gui.exe"; Description: "Launch VaptVupt"; \
+  Flags: nowait postinstall skipifsilent
+
+[Code]
+function NeedsAddPath(Param: string): Boolean;
+var
+  OrigPath: string;
+begin
+  if not RegQueryStringValue(HKEY_CURRENT_USER, 'Environment', 'Path', OrigPath) then
+  begin
+    Result := True;
+    exit;
+  end;
+  Result := Pos(';' + ExpandConstant(Param) + ';', ';' + OrigPath + ';') = 0;
+end;