CRITICAL FIX: Eliminate all hardcoded /tmp paths - respect WorkDir configuration

This is a critical bugfix release addressing multiple hardcoded temporary directory paths
that prevented proper use of the WorkDir configuration option.

PROBLEM:
Users configuring WorkDir (e.g., /u01/dba/tmp) for systems with small root filesystems
still experienced failures because critical operations hardcoded /tmp instead of respecting
the configured WorkDir. This made the WorkDir option essentially non-functional.

FIXED LOCATIONS:
1. internal/restore/engine.go:632 - CRITICAL: Used BackupDir instead of WorkDir for extraction
2. cmd/restore.go:354,834 - CLI restore/diagnose commands ignored WorkDir
3. cmd/migrate.go:208,347 - Migration commands hardcoded /tmp
4. internal/migrate/engine.go:120 - Migration engine ignored WorkDir
5. internal/config/config.go:224 - SwapFilePath hardcoded /tmp
6. internal/config/config.go:519 - Backup directory fallback hardcoded /tmp
7. internal/tui/restore_exec.go:161 - Debug logs hardcoded /tmp
8. internal/tui/settings.go:805 - Directory browser default hardcoded /tmp
9. internal/tui/restore_preview.go:474 - Display message hardcoded /tmp

NEW FEATURES:
- Added Config.GetEffectiveWorkDir() helper method
- WorkDir now respects WORK_DIR environment variable
- All temp operations now consistently use configured WorkDir with /tmp fallback

IMPACT:
- Restores on systems with small root disks now work properly with WorkDir configured
- Admins can control disk space usage for all temporary operations
- Debug logs, extraction dirs, swap files all respect WorkDir setting

Version: 3.42.1 (Critical Fix Release)

internal/config/config.go

@@ -223,8 +223,11 @@ func New() *Config {
 		// Cluster parallelism (default: 2 concurrent operations for faster cluster backup/restore)
 		ClusterParallelism: getEnvInt("CLUSTER_PARALLELISM", 2),
 
+		// Working directory for large operations (default: system temp)
+		WorkDir: getEnvString("WORK_DIR", ""),
+
 		// Swap file management
-		SwapFilePath:   getEnvString("SWAP_FILE_PATH", "/tmp/dbbackup_swap"),
+		SwapFilePath:   "", // Will be set after WorkDir is initialized
 		SwapFileSizeGB: getEnvInt("SWAP_FILE_SIZE_GB", 0), // 0 = disabled by default
 		AutoSwap:       getEnvBool("AUTO_SWAP", false),
 
@@ -264,6 +267,13 @@ func New() *Config {
 		cfg.SSLMode = "prefer"
 	}
 
+	// Set SwapFilePath using WorkDir if not explicitly set via env var
+	if envSwap := os.Getenv("SWAP_FILE_PATH"); envSwap != "" {
+		cfg.SwapFilePath = envSwap
+	} else {
+		cfg.SwapFilePath = filepath.Join(cfg.GetEffectiveWorkDir(), "dbbackup_swap")
+	}
+
 	return cfg
 }
 
@@ -499,6 +509,14 @@ func GetCurrentOSUser() string {
 	return getCurrentUser()
 }
 
+// GetEffectiveWorkDir returns the configured WorkDir or system temp as fallback
+func (c *Config) GetEffectiveWorkDir() string {
+	if c.WorkDir != "" {
+		return c.WorkDir
+	}
+	return os.TempDir()
+}
+
 func getDefaultBackupDir() string {
 	// Try to create a sensible default backup directory
 	homeDir, _ := os.UserHomeDir()
@@ -516,7 +534,7 @@ func getDefaultBackupDir() string {
 		return "/var/lib/pgsql/pg_backups"
 	}
 
-	return "/tmp/db_backups"
+	return filepath.Join(os.TempDir(), "db_backups")
 }
 
 // CPU-related helper functions