feat: KVM port management — configuration, switching, and navigation shell

Backend (crates/ericrfb-proxy):
- Session cookie now persisted in AppState for device API calls
- New kvm.rs with three REST endpoints:
  GET /api/kvm/ports — scrapes kvm.asp, returns port config as JSON
  PUT /api/kvm/ports — saves port names, hotkeys, visibility, count
  POST /api/kvm/switch — switches active KVM port via home2.asp
- HTML scraping extracts form values from predictable firmware HTML

Frontend (crates/ericrfb-frontend):
- New shell.ts: sidebar navigation with page routing pattern
  (Console, Ports — extensible for Virtual Media, Users, etc.)
- Console refactored into pages/console.ts with mount/unmount lifecycle
  - Port switcher dropdown in toolbar (fetches port list, switches on change)
  - WebSocket auto-reconnects after port switch
- New pages/ports.ts: editable port configuration table
  - Port count, key pause duration, per-port name/hotkey/show-in-console
  - Save, reload, and per-port switch buttons
  - Active port highlighted
- Dark theme sidebar with active state indicators

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/ericrfb-proxy/Cargo.toml

@@ -10,6 +10,7 @@ tokio.workspace = true
 axum.workspace = true
 reqwest.workspace = true
 serde.workspace = true
+serde_json = "1"
 toml.workspace = true
 tower-http.workspace = true
 tracing.workspace = true