Unify OpenNOW GFN identity to desktop everywhere

[zortos293]

This PR unifies all GeForce NOW client-facing requests to identify as the Windows desktop GFN client (native/NVIDIA-CLASSIC) instead of mixing browser/web personas and platform-specific identities.

New shared identity module (src/main/gfn/clientIdentity.ts)

• Added canonical desktop GFN constants: GFN-PC, NATIVE, NVIDIA-CLASSIC, WINDOWS, desktop UA, origin/referer
• Added buildDesktopGfnHeaders() helper for consistent request headers with optional token, origin, browser-type, and device details
• Added buildDesktopCloudMatchIdentity() helper for session body identity fields

Refactored modules

• auth.ts: getRegions() and getVpcId() now use desktop NATIVE/NVIDIA-CLASSIC headers instead of BROWSER/WEBRTC
• subscription.ts: fetchSubscription() and fetchDynamicRegions() now use shared desktop headers
• games.ts: GraphQL requests and VPC lookup now use shared desktop headers and GFN-PC constant
• cloudmatch.ts: Request headers centralized through shared builder, removing process.platform OS drift; session-create and session-resume bodies use shared identity helpers
• signaling.ts: WebSocket handshake now uses desktop GFN UA and origin constants

Switched GFN-facing paths to desktop/native identity

• MES API (/v4/subscriptions) now presents as desktop NATIVE/NVIDIA-CLASSIC
• Dynamic regions (/v2/serverInfo) in auth.ts now presents as desktop NATIVE/NVIDIA-CLASSIC
• CloudMatch create, poll, and claim requests now use uniform Windows desktop headers with no platform branching
• Signaling websocket now presents as desktop GFN client

Intentional exception

• Kept CloudMatch GSStreamerType: "WebRTC" as transport metadata (documented in shared module) since it describes the negotiated stream, not the client persona