Refactor: split session.rs (1162 lines) into focused modules

[CREVIOS]

Problem

`src-tauri/src/rdp/session.rs` is 1,162 lines — a god module containing:

• `SessionActor` struct and all its methods
• `SessionManager` struct (registry + lifecycle)
• `PerformanceMetrics` struct
• `SessionState` enum
• `SessionInfo` struct (serialized to frontend)
• Mock session loop
• Real RDP session loop
• Frame emission logic
• Input forwarding
• Clipboard message flushing
• Auto-reconnect with backoff
• Helper functions (`normalize_remote_resolution`)

This makes it hard for contributors to understand the codebase and find where to make changes.

Proposed split

New file	Contents	~Lines
`session/mod.rs`	Re-exports, `SessionState`, `SessionInfo`, `PerformanceMetrics`	~80
`session/actor.rs`	`SessionActor` struct + `new()` + `run()`	~200
`session/rdp_loop.rs`	`run_real_session_loop()` — the main PDU processing loop	~300
`session/mock_loop.rs`	`run_mock_session_loop()` — for testing without a server	~80
`session/manager.rs`	`SessionManager` — registry, create/remove/list sessions	~200
`session/reconnect.rs`	Auto-reconnect logic with exponential backoff	~150

Rules for the split

• No behavior changes — pure refactor
• All public APIs remain the same
• `SessionActor` fields stay private (accessed via methods)
• Use `pub(crate)` for inter-module visibility
• Run `cargo test` before and after to verify no regressions

Files to modify

• `src-tauri/src/rdp/session.rs` → split into `src-tauri/src/rdp/session/` directory
• `src-tauri/src/rdp/mod.rs` — update module declaration

Priority: P2 — improves contributor experience