Multi-session GPU rendering — switch SharedFrame per active tab

[CREVIOS]

Problem

When multiple RDP sessions are open (tabs), the GpuRenderer only holds a reference to ONE SharedFrame. Switching tabs doesn't change which session's frames are rendered by the GPU.

Current architecture

```
SessionActor 1 → SharedFrame A (global, managed state)
SessionActor 2 → SharedFrame A (same one!) ← WRONG
GpuRenderer → SharedFrame A
```

Both sessions write to the same SharedFrame, causing frame corruption (interleaved pixels from different desktops).

Correct architecture

```
SessionActor 1 → SharedFrame A (per-session)
SessionActor 2 → SharedFrame B (per-session)
GpuRenderer → active_frame (switches on tab change)

Tab switch: user clicks "Session 2" tab
→ frontend sends IPC: set_active_session(session_2_id)
→ GpuRenderer.set_active_frame(SharedFrame B)
→ next render() uses SharedFrame B
```

Implementation

Step 1: Per-session SharedFrame

In `SessionManager::create_session()`, create a NEW SharedFrame for each session instead of using the global one:

```rust
let shared_frame = SharedFrame::new(width, height);
// Store in the session handle
```

Step 2: SharedFrame registry

Add a `HashMap<String, Arc>` to managed state, keyed by session ID:

```rust
pub struct FrameRegistry {
frames: Mutex<HashMap<String, Arc>>,
active: Mutex<Option<Arc>>,
}
```

Step 3: Active frame switching

Add a Tauri command `set_active_session(session_id)` that:

1. Looks up the session's SharedFrame in the registry
2. Swaps it into `GpuRenderer`'s `shared_frame` field
3. Forces a full repaint

Step 4: Frontend integration

In `SessionView.tsx`, when switching tabs, call `set_active_session(newSessionId)`.

Files to modify

• `src-tauri/src/renderer/shared_frame.rs` — no changes needed
• `src-tauri/src/renderer/gpu.rs` — make `shared_frame` swappable (`Arc` or `Mutex<Arc>`)
• `src-tauri/src/rdp/session.rs` — per-session SharedFrame creation
• `src-tauri/src/lib.rs` — manage `FrameRegistry` instead of single SharedFrame
• `src-tauri/src/commands/session.rs` — add `set_active_session` command
• `src/components/session/SessionView.tsx` — call `set_active_session` on tab switch

Priority: P1 — multi-session is broken without this