feat(update): one-click desktop self-update (AppImage, .deb, Windows)

update_apply used to flatly refuse to run for the GUI binary and point
the user to a manual download instead. Since both desktop apps already
elevate to root/administrator before showing their window, the
process applying the update already has everything it needs — no
second authentication round is required.

- New guara_core::update::apply_desktop[_with_progress], alongside the
  existing CLI-only apply() (left untouched): detects the install kind
  at runtime (AppImage via the APPIMAGE env var — current_exe() is
  unreliable there, same FUSE-mount issue elevated_exe() already works
  around and this reuses; Deb otherwise on Linux; WindowsInstaller on
  Windows), matches the right release asset by the GUI bundle naming
  (_amd64/_aarch64.AppImage, _amd64/_arm64.deb, _x64-setup.exe — a
  different suffix convention than the CLI tarballs), verifies SHA-256
  when a sidecar is published, and installs:
  - AppImage: downloads to a temp file in the same directory (same
    filesystem, atomic rename), renames over the running file, execs
    it in place (same PID, same already-root privileges — no elevation
    dance needed).
  - .deb: runs `dpkg -i` directly (already root), then execs
    current_exe() again now that dpkg replaced it on disk.
  - Windows: runs the downloaded installer with /S (silent) detached,
    then exits immediately to release the file lock on our own .exe.
    Does NOT claim to relaunch the app afterwards — there is no
    documented NSIS or Tauri flag for that (verified against NSIS's
    and Tauri's own docs: Tauri's own updater restarts the app from its
    own Rust code after the installer exits, not via a command-line
    switch), and blocking here on the installer's exit status risks a
    self-deadlock if its script waits for this process to close before
    it can overwrite the running .exe. Needs a real Windows machine to
    confirm the actual post-install behavior.
- Engine::update_apply_desktop wires it into the Tauri `update_apply`
  command, which now also takes AppHandle and emits `update-progress`
  events (downloading/verifying/installing/restarting) for the UI.
- Sabiá: the existing #btnUpdate handler changed the button's label to
  "update to X" but never actually called apply — wired it for real,
  with the same progress feedback.
- Guará: restored the "Update now" action in the update card (removed
  during the updater-unification migration), wired to the new command.
- scripts/build-gui.sh: generates a .sha256 next to every file in
  dist-gui/, matching the existing convention for CLI release
  archives, so apply_desktop can verify downloads the same way CLI
  update --apply already does.
- 3 new unit tests per repo: install-kind detection, GUI asset suffix
  matching against real release naming, and the existing CLI-refusal
  test still passes unchanged.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VAyr8d7HuAdUGB6aA5xmDD

apps/gui/ui/i18n-en.js

@@ -582,7 +582,7 @@ export const EN = {
   'Atualizando o Guará…': 'Updating Guará…',
   'Baixando a atualização…': 'Downloading the update…',
   'Baixando… {n}%': 'Downloading… {n}%',
-  'Instalando… o Guará vai reabrir sozinho.': 'Installing… Guará will reopen by itself.',
+  'Instalando a atualização…': 'Installing the update…',
   'Esta instalação vem de um pacote do sistema e não se atualiza sozinha. Baixe a versão nova na página da release:': 'This install comes from a system package and cannot update itself. Download the new version from the release page:',
   'Atualizações automáticas': 'Automatic updates',
   'Ao abrir, consulta o Gitea para ver se há versão nova (uma consulta, sem enviar dados seus)': 'On startup, checks Gitea for a new version (one request, nothing of yours is sent)',
@@ -593,4 +593,9 @@ export const EN = {
   'Nova versão {n} disponível': 'New version {n} available',
   'Você já está na versão mais recente': 'You are already on the latest version',
   'Não foi possível verificar atualizações: {n}': 'Could not check for updates: {n}',
+  'Instalação automática da nova versão com um clique:': 'One-click automatic installation of the new version:',
+  'Verificando integridade…': 'Verifying checksum…',
+  'Reiniciando…': 'Restarting…',
+  'Atualização concluída': 'Update completed',
+  'Falha na atualização: {n}': 'Update failed: {n}',
 };