Add tray context menu for manual mouse model selection (MX Master 3S / MX Vertical)

[Copilot]

Without a manual override, users whose device isn't auto-detected via HID++ have no way to tell Mouser which mouse they're using, leaving the UI in the wrong state (e.g., gesture/hscroll hotspots shown for MX Vertical users).

Changes

core/config.py

• Added selected_device: "" to DEFAULT_CONFIG settings — persists manual choice across restarts; empty string means HID++ auto-detect only

ui/backend.py

• deviceName — auto-detected name takes precedence; falls back to selected_device when no HID++ device is active
• deviceDisplayName — delegates to deviceName, ultimately falls back to "MX Master 3S"
• selectedDevice property — exposes the config setting to QML
• setSelectedDevice(name) slot — saves to config and emits settingsChanged + deviceChanged, updating all dependent bindings (title bar, tray tooltip, isMxVertical in MousePage.qml)

main_qml.py

• Added "Select Mouse" submenu to the system tray context menu with checkable "MX Master 3S" and "MX Vertical" actions
• Check state stays in sync via backend.settingsChanged

# Tray menu structure
Open Settings
Select Mouse ▶  ✓ MX Master 3S
                  MX Vertical
─────────────────
Disable Remapping
...
Quit Mouser

The existing isMxVertical binding in MousePage.qml (backend.deviceName === "MX Vertical") requires no changes — it correctly reacts to both auto-detected and manually selected device names.

Original prompt

Pull request: #4
add contextual menu to select mouse between Mx master 3s and Mx vertical

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