feat(install): offer one-click sync engine installation from the app

[gnacho]

What

Today, installing the sync engine requires leaving the app for a terminal. This adds one-click installation from the two places that already report the missing engine (#209, #210):

• Pure core (src/core/engine_install.rs): maps (provider, detected package manager) to an exact install plan - a fixed pkexec <pkg-manager> … argv (no shell, no user input) plus a display/copyable command. Package names verified against the distro indexes: nextcloud-client (pacman/dnf/zypper), nextcloud-desktop-cmd (apt, provides nextcloudcmd), opencloud-desktop (pacman; AUR fallback hint on failure). Unsupported combinations (OpenCloud on apt/dnf/zypper) fall back to manual guidance.
• Wizard: the blocking "Sync Engine Not Installed" dialog now offers Install; on success it confirms the engine landed and Finish proceeds.
• Folder row: the ⋯ menu leads with "Install sync engine…" while the provider's binary is missing; a successful install re-checks binary_exists() and triggers a manual retry so the folder recovers on its own.
• Failure path shows the exit code, the stderr tail and the manual command (with copy). No background installs, no sudo loops, single guarded pkexec run.

Tests

• 11 new core tests (detection priority, argv exactness per combo, safety invariants: single tokens, no paths, no engine binary names in argv).
• 3 pure dialog-text tests (automatic/manual/failure bodies).
• 1 GTK smoke test for the row menu entry gating.

Suite: 731 passed + 1 ignored (known flaky login_flow), clippy -D warnings clean, fmt clean, i18n parity regenerated (525 entries).

Closes #218