feat: implement Phase 2 (Go Agent) and Phase 3 (React Frontend MVP)

Phase 2 - Go Agent Core:
- gRPC client with exponential backoff reconnect logic
- Command executor (PowerShell/sh cross-platform)
- Proto stubs regenerated with module= option (correct output path)
- gRPC upgraded to v1.79.3 (BidiStreamingClient support)

Phase 3 - React Frontend MVP:
- Vite + React 18 + TypeScript setup with Tailwind CSS v4
- TanStack Query for data fetching, API client + TypeScript types
- Dashboard page: stats cards (agents/status/tickets) + sortable agents table
- Agent detail page: CPU/RAM charts (Recharts), disk usage, shell command executor
- Tickets page: CRUD with modals, filters, sortable table
- Dark mode with CSS custom properties

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

Agent/internal/connection/grpc_client.go

@@ -0,0 +1,83 @@
+package connection
+
+import (
+	"context"
+	"fmt"
+	"log"
+	"time"
+
+	pb "nexusrmm.local/agent/pkg/proto"
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/connectivity"
+	"google.golang.org/grpc/credentials/insecure"
+	"google.golang.org/grpc/keepalive"
+)
+
+type GrpcClient struct {
+	conn    *grpc.ClientConn
+	Client  pb.AgentServiceClient
+	address string
+}
+
+func NewGrpcClient(address string) (*GrpcClient, error) {
+	opts := []grpc.DialOption{
+		grpc.WithTransportCredentials(insecure.NewCredentials()),
+		grpc.WithKeepaliveParams(keepalive.ClientParameters{
+			Time:                30 * time.Second,
+			Timeout:             10 * time.Second,
+			PermitWithoutStream: true,
+		}),
+	}
+
+	conn, err := grpc.NewClient(address, opts...)
+	if err != nil {
+		return nil, err
+	}
+
+	return &GrpcClient{
+		conn:    conn,
+		Client:  pb.NewAgentServiceClient(conn),
+		address: address,
+	}, nil
+}
+
+func (g *GrpcClient) Close() {
+	if g.conn != nil {
+		g.conn.Close()
+	}
+}
+
+func ConnectWithRetry(address string, maxRetries int) (*GrpcClient, error) {
+	for i := 0; i < maxRetries; i++ {
+		client, err := NewGrpcClient(address)
+		if err != nil {
+			log.Printf("Client creation %d/%d failed: %v", i+1, maxRetries, err)
+			time.Sleep(backoffDuration(i))
+			continue
+		}
+
+		ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+		client.conn.Connect()
+		state := client.conn.GetState()
+		client.conn.WaitForStateChange(ctx, state)
+		cancel()
+
+		newState := client.conn.GetState()
+		if newState == connectivity.Ready || newState == connectivity.Idle {
+			return client, nil
+		}
+
+		log.Printf("Connection attempt %d/%d: state=%v", i+1, maxRetries, newState)
+		client.Close()
+		time.Sleep(backoffDuration(i))
+	}
+	return nil, fmt.Errorf("failed to connect after %d attempts", maxRetries)
+}
+
+func backoffDuration(attempt int) time.Duration {
+	secs := 2 << attempt
+	if secs > 30 {
+		secs = 30
+	}
+	return time.Duration(secs) * time.Second
+}

Agent/internal/executor/executor.go

@@ -0,0 +1,49 @@
+package executor
+
+import (
+	"bytes"
+	"context"
+	"os/exec"
+	"runtime"
+	"time"
+)
+
+type Result struct {
+	ExitCode int
+	Stdout   string
+	Stderr   string
+	Success  bool
+}
+
+func Execute(ctx context.Context, command string, timeoutSec int) *Result {
+	if timeoutSec <= 0 {
+		timeoutSec = 300
+	}
+	ctx, cancel := context.WithTimeout(ctx, time.Duration(timeoutSec)*time.Second)
+	defer cancel()
+
+	var cmd *exec.Cmd
+	if runtime.GOOS == "windows" {
+		cmd = exec.CommandContext(ctx, "powershell", "-NoProfile", "-NonInteractive", "-Command", command)
+	} else {
+		cmd = exec.CommandContext(ctx, "/bin/sh", "-c", command)
+	}
+
+	var stdout, stderr bytes.Buffer
+	cmd.Stdout = &stdout
+	cmd.Stderr = &stderr
+
+	err := cmd.Run()
+	result := &Result{
+		Stdout:  stdout.String(),
+		Stderr:  stderr.String(),
+		Success: err == nil,
+	}
+	if cmd.ProcessState != nil {
+		result.ExitCode = cmd.ProcessState.ExitCode()
+	}
+	if err != nil && result.ExitCode == 0 {
+		result.ExitCode = -1
+	}
+	return result
+}