feat(v0.2.0): config UI panel + folder picker + S3 bucket lister/tester

- Panel Reglages dans l'UI (modal)
- Folder picker natif Wails pour dossier local
- ListS3Buckets via aws-cli (profile-aware)
- TestS3Access pour verifier acces bucket
- Config persistee user config dir (cross-OS): %APPDATA%/field-sync ou ~/.config/field-sync
- Header affiche bucket + dest visibles
- Auto-open modal si bucket vide au premier run

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/config/config.go

@@ -1,7 +1,9 @@
 package config
 
 import (
+	"encoding/json"
 	"os"
+	"path/filepath"
 	"strconv"
 
 	"github.com/joho/godotenv"
@@ -15,7 +17,52 @@ type Config struct {
 	Concurrency int    `json:"concurrency"`
 }
 
-// Load reads config from ENV (with optional .env file)
+// Path returns the cross-OS config file path
+func Path() string {
+	dir, _ := os.UserConfigDir()
+	return filepath.Join(dir, "field-sync", "config.json")
+}
+
+// Save writes config to disk (atomic)
+func Save(cfg Config) error {
+	p := Path()
+	if err := os.MkdirAll(filepath.Dir(p), 0755); err != nil {
+		return err
+	}
+	data, _ := json.MarshalIndent(cfg, "", "  ")
+	tmp := p + ".tmp"
+	if err := os.WriteFile(tmp, data, 0644); err != nil {
+		return err
+	}
+	return os.Rename(tmp, p)
+}
+
+// LoadFromFile reads from JSON config file (no .env)
+func LoadFromFile() Config {
+	data, err := os.ReadFile(Path())
+	if err != nil {
+		return Config{AWSProfile: "cosma", Concurrency: 4, LocalDest: defaultLocalDest()}
+	}
+	var c Config
+	json.Unmarshal(data, &c)
+	if c.Concurrency == 0 {
+		c.Concurrency = 4
+	}
+	if c.AWSProfile == "" {
+		c.AWSProfile = "cosma"
+	}
+	if c.LocalDest == "" {
+		c.LocalDest = defaultLocalDest()
+	}
+	return c
+}
+
+func defaultLocalDest() string {
+	home, _ := os.UserHomeDir()
+	return filepath.Join(home, "field-sync-data")
+}
+
+// Load reads config from ENV (with optional .env file) — kept for compat
 func Load(envFile string) Config {
 	_ = godotenv.Load(envFile) // ignore if missing