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/migrate/engine.go

@@ -117,7 +117,7 @@ func NewEngine(sourceCfg, targetCfg *config.Config, log logger.Logger) (*Engine,
 		targetDB:    targetDB,
 		log:         log,
 		progress:    progress.NewSpinner(),
-		workDir:     os.TempDir(),
+		workDir:     sourceCfg.GetEffectiveWorkDir(),
 		keepBackup:  false,
 		jobs:        4,
 		dryRun:      false,