fix(sync): detect a missing sync engine and stop reporting false success (closes #209)

[gnacho]

Problem

nextsync delegates reconciliation to an external engine (nextcloudcmd / opencloudcmd). When that binary is not on $PATH:

• real runs ended in the generic Failed ("Synchronization failed - view the log"), and
• a periodic remote-interval run whose root ETag was unchanged took the ETag-gate shortcut (EngineRun::Direct(SyncOutcome::Success)) without ever resolving the engine - so the row reported "Synchronization completed" for days while nothing was actually synchronized.

The concrete trigger: removing the official Nextcloud desktop package (e.g. to stop its D-Bus/Nautilus auto-start, #208) also removes nextcloudcmd, and the app kept looking healthy.

Fix

• engine_thread now builds the engine command before the remote ensurer and the ETag gate, mapping CommandError::MissingBinary to the new distinct outcome SyncOutcome::EngineMissing (other command errors keep the generic Failed).
• resolve_binary treats an explicit executable override pointing at a path that no longer exists as MissingBinary too (a stale configured path surfaces the actionable state instead of an opaque spawn failure).
• The scheduler maps EngineMissing to AppState::Error with a clear message ("Synchronization blocked: the sync engine is not installed"), never counts it as a success, and keeps retrying on every trigger - installing the engine package recovers without an app restart.
• The outcome line goes to the daily log (outcome_log_line) and raises a desktop notification (failure_notification), so "view the log" is meaningful.
• Spanish translations added; i18n catalog parity regenerated.

Tests (red-green)

• missing_engine_blocks_the_etag_gate_success - unchanged ETag + missing binary must not report Success (was: Success without spawning anything).
• missing_engine_reports_engine_missing_on_manual_run - manual run with a missing engine reports EngineMissing (was: Failed).
• missing_binary_maps_to_failed updated: now expects EngineMissing (the old behavior this issue changes).
• engine_missing_sets_error_and_recovers_on_retry - scheduler state, message mentions the engine, and recovery on the next trigger.
• resolve_binary_rejects_a_missing_explicit_override, notifications coverage, outcome-log coverage.

Gate: 712 passed + 1 ignored (known flaky), clippy --all-targets clean, fmt clean, i18n parity diff clean.

Note

Local package built and running (0.2.22-1). With this merged, #208 becomes safe to execute.