Platform selection bug: darwin-arm64 extensions served instead of linux-x64 due to alphabetical sorting

## Bug Description

When using the `/api/vscode/{publisher}/{extension}/latest` endpoint (introduced in v2.4.0), the marketplace incorrectly serves `darwin-arm64` extension variants instead of the appropriate platform-specific versions (e.g., `linux-x64`) due to alphabetical sorting of platform names.

## Root Cause

The issue is in the `ByVersion.Less()` method in `/storage/storage.go`:

```go
if vs[i].Version == vs[j].Version {
return vs[i].TargetPlatform < vs[j].TargetPlatform
}
```

When multiple platform variants exist for the same semantic version (e.g., `2.36.4@darwin-arm64`, `2.36.4@linux-x64`), they are sorted alphabetically by platform string. Since `"darwin-arm64" < "linux-x64"` alphabetically, darwin-arm64 versions are always selected first when using `IncludeLatestVersionOnly`.

## Impact

- Extensions fail to install in VS Code/code-server running on Linux platforms
- Affects all extensions that have multiple platform-specific variants
- Introduced in v2.4.0 when the new `/latest` endpoint started being used by VS Code

## Example Logs

**Expected behavior (linux-x64 environment should get linux-x64 variant):**
```
ComputeTargetPlatform: linux-x64
2025-09-20 09:42:42.565 [debu] GET /files/hashicorp/terraform/2.36.4@linux-x64/hashicorp.terraform-2.36.4@linux-x64.vsix
```

**Actual behavior (linux-x64 environment incorrectly gets darwin-arm64 variant):**
```
ComputeTargetPlatform: linux-x64
2025-09-20 09:42:40.247 [debu] GET /files/hashicorp/terraform/2.36.4@darwin-arm64/extension/package.json
```

## Environment

- code-marketplace version: v2.4.0
- VS Code/code-server: 1.104.0
- Platform: Linux x64 containers
- Extensions affected: Any extension with multiple platform variants (e.g., HashiCorp Terraform)

## Proposed Solutions

1. **Platform-aware filtering**: Filter versions by the requesting client's target platform before sorting
2. **Universal version priority**: Prioritize universal platform versions when available
3. **Platform preference order**: Define a logical platform ordering instead of alphabetical (e.g., prefer common platforms like linux-x64 over darwin-arm64)
4. **Client platform detection**: Extract target platform from request headers or query parameters

## Reproduction Steps

1. Set up code-marketplace v2.4.0
2. Upload an extension with multiple platform variants (e.g., versions for both `linux-x64` and `darwin-arm64`)
3. Make a request to `/api/vscode/{publisher}/{extension}/latest` from a linux-x64 environment
4. Observe that darwin-arm64 variant is returned instead of linux-x64

## Workaround

Temporarily downgrade to v2.3.1 or modify the version sorting logic to prioritize the appropriate platform for your environment.