Epic #74.4: SRP refactor desktop-tauri backend main

[ppotepa]

Parent: #74

Objective

Split hotspot file into smaller modules/components with single-responsibility boundaries.

Target

• File: desktop/apps/desktop-tauri/src-tauri/src/main.rs
• Stack: Rust/Tauri
• Current size: ~2285 LOC

Scope

• Extract cohesive domains/functions into dedicated files/modules.
• Keep public behavior and contracts stable.
• Reduce file responsibility breadth and coupling.

Definition of Done

• Main hotspot file reduced and responsibilities separated.
• Existing checks/tests/smoke flows remain green.
• No functional regression in runtime behavior.