Fix: IPC frame send failure kills session when GPU renderer is active

[CREVIOS]

Problem

In `session.rs`, the IPC frame path runs ALWAYS — even when the GPU renderer is active and handling display. If the IPC channel closes or errors, `send_frame_packet()` returns `Err` and the session disconnects with `DisconnectReason::ConnectionLost`.

This is wrong: when the GPU renderer is the primary display path, IPC frame failures should be silently ignored (the user is seeing frames via GPU).

Current code (session.rs ~line 583)

```rust
// Always send via IPC — webview Canvas renders these
let packet = encode_image_update_packet(...);
if self.send_frame_packet(packet).is_err() {
return DisconnectReason::ConnectionLost; // ← KILLS SESSION
}
```

Fix

```rust
if self.shared_frame.is_some() {
// GPU renderer is primary — IPC is optional, don't kill session on failure
let _ = self.send_frame_packet(packet);
} else {
// IPC is the only display path — failure is fatal
if self.send_frame_packet(packet).is_err() {
return DisconnectReason::ConnectionLost;
}
}
```

Or even better: skip IPC entirely when GPU is active to save CPU:

```rust
if self.shared_frame.is_none() {
let packet = encode_image_update_packet(...);
if self.send_frame_packet(packet).is_err() {
return DisconnectReason::ConnectionLost;
}
}
```

This requires issue #14 (transparent webview) to be implemented first, otherwise the screen is black.

Depends on

• Make wgpu GPU-rendered frames visible via transparent webview overlay #14 (transparent webview overlay)

Files to modify

• `src-tauri/src/rdp/session.rs` — conditional IPC send

Priority: P1 — prevents false disconnections