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>

app.go

@@ -3,6 +3,8 @@ package main
 import (
 	"context"
 	"fmt"
+	"os/exec"
+	"strings"
 	"os"
 	"path/filepath"
 	"sync"
@@ -66,12 +68,67 @@ func NewApp() *App {
 // startup is called by Wails when the app starts
 func (a *App) startup(ctx context.Context) {
 	a.ctx = ctx
-	a.cfg = config.Load(".env")
+	a.cfg = config.LoadFromFile()
 	for i, letter := range []string{"A", "B", "C", "D"} {
 		a.slots[i] = SlotState{Slot: letter, Status: "idle"}
 	}
 }
 
+// SaveConfig persists config to user config dir and updates in-memory config
+func (a *App) SaveConfig(cfg config.Config) error {
+	if err := config.Save(cfg); err != nil {
+		return err
+	}
+	a.mu.Lock()
+	a.cfg = cfg
+	a.mu.Unlock()
+	return nil
+}
+
+// GetConfigPath returns where the config is stored (for display)
+func (a *App) GetConfigPath() string {
+	return config.Path()
+}
+
+// PickLocalDest opens native folder picker, returns selected path or "" if cancelled
+func (a *App) PickLocalDest() (string, error) {
+	path, err := runtime.OpenDirectoryDialog(a.ctx, runtime.OpenDialogOptions{
+		Title: "Dossier de destination locale",
+	})
+	return path, err
+}
+
+// ListS3Buckets runs `aws s3api list-buckets --profile <p>` returns []string
+func (a *App) ListS3Buckets(profile string) ([]string, error) {
+	args := []string{"s3api", "list-buckets", "--query", "Buckets[].Name", "--output", "text"}
+	if profile != "" {
+		args = append(args, "--profile", profile)
+	}
+	out, err := exec.Command("aws", args...).CombinedOutput()
+	if err != nil {
+		return nil, fmt.Errorf("aws error: %s", strings.TrimSpace(string(out)))
+	}
+	raw := strings.TrimSpace(string(out))
+	if raw == "" || raw == "None" {
+		return []string{}, nil
+	}
+	parts := strings.Fields(raw)
+	return parts, nil
+}
+
+// TestS3Access checks bucket reachable: `aws s3 ls s3://<bucket>/ --profile <p>`
+func (a *App) TestS3Access(bucket, profile string) error {
+	args := []string{"s3", "ls", "s3://" + bucket + "/"}
+	if profile != "" {
+		args = append(args, "--profile", profile)
+	}
+	out, err := exec.Command("aws", args...).CombinedOutput()
+	if err != nil {
+		return fmt.Errorf("accès refusé: %s", strings.TrimSpace(string(out)))
+	}
+	return nil
+}
+
 // ScanCards returns up to 4 detected SD cards
 func (a *App) ScanCards() []detect.SDCard {
 	cards := detect.Detect()