fix: accept zero retention limits, retire Store() for typed accessors

Phase 8 (PROJECT_REVIEW_PLAN.md 8.1): 0 in MaxLogFiles/MaxLogAgeDays now
means "keep everything" end to end. runner.CleanupLogs already treated
<= 0 as disabled; validateConfig, the Settings form, and
loadOrCreateConfig's backfill were the only things making that state
unreachable.

Phase 9 (1.1, rolling up 1.2, 1.3, 7.3): added Service.Config() and
Service.Paths(), copying under mu, and converted every UI site that read
Service state through the raw *storage.Store returned by Store() (now
removed). jobs_view's pause control is now driven by refreshView reading
svc.Config().Paused on every event instead of only mirroring its own tap
handler, which makes it an actual consumer of SchedulerStateChanged.
mainwindow's event listener is a real type switch, and events.go's doc
comment no longer claims a compiler exhaustiveness check Go doesn't have.
Unexported the redundant SetAutostart/AutostartStatus package functions
in platform/autostart now that only the Manager methods are used outside
the package.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/operations.go

@@ -503,11 +503,13 @@ func validateConfig(config domain.Config) error {
 	if strings.TrimSpace(config.LogsDir) == "" {
 		return errors.New("logs directory is required")
 	}
-	if config.MaxLogFiles <= 0 {
-		return errors.New("max log files must be a positive number")
+	// 0 means "keep everything" (see runner.CleanupLogs); only a negative count
+	// is rejected, the same three-state shape as DefaultTimeoutSeconds below.
+	if config.MaxLogFiles < 0 {
+		return errors.New("max log files must be zero (unlimited) or a positive number")
 	}
-	if config.MaxLogAgeDays <= 0 {
-		return errors.New("max log age days must be a positive number")
+	if config.MaxLogAgeDays < 0 {
+		return errors.New("max log age days must be zero (unlimited) or a positive number")
 	}
 	if config.ExecutionMode != domain.ExecutionModeParallel && config.ExecutionMode != domain.ExecutionModeSequential {
 		return errors.New("execution mode must be 'parallel' or 'sequential'")