feat(desktop): keep PearPass running in system tray when window is closed

[andrey-dru-me1]

Closes #561

Keep PearPass running in the tray when the window is closed, so the browser-extension connection stays alive without a visible window taking up space.

• Intercept window close: hide to tray instead of quitting
• Tray icon with Show/Quit context menu
• Settings toggle persisted via devicePreferences.json
• macOS dock show/hide on tray interaction
• Refactor devicePreferences to generic boolean-key pattern

Requirements

• The browser extension must remain connected even when no PearPass window is visible.
• Closing the window (red X) must not terminate the app process.
• Users need an explicit "Quit" action in the tray to fully exit.
• The behavior must be opt-in via a setting (off by default).

Changes

• electron/main.cjs — added Tray creation, window-close interception (win.on('close')), forceQuit flag, backgroundModeEnabled state, and IPC handlers for show/hide/quitting.
• electron/preload.cjs — exposed backgroundModeEnabled to the renderer.
• src/utils/devicePreferences.cjs — generalized from a single loggingEnabled key to a DEFAULTS map with coerce() for any future boolean preferences. Added backgroundModeEnabled: false default.
• src/utils/devicePreferences.test.js — updated tests for the new generic key pattern.
• src/pages/SettingsView/content/AppPreferencesContent/ — added a "Keep running in background" toggle that reads/writes backgroundModeEnabled via the preload bridge.
• src/types/electron.d.ts — added backgroundModeEnabled to the ElectronAPI type.
• src/constants/localStorage.js — added BACKGROUND_MODE_ENABLED constant.
• Snapshot updates (7 files) reflecting the toggle addition.

Testing Notes

• Manually tested on macOS with the toggle on/off, verifying close-to-tray behavior, tray menu Show/Quit, and dock icon visibility.
• Unit tests for devicePreferences updated and passing.
• Verified browser extension stays connected when window is in tray.

Things reviewers should pay attention to

• The forceQuit flag pattern to distinguish close-vs-quit on macOS — check that app.quit() and app.exit() are used in the right places.
• The coerce() function in devicePreferences — this changes the write path for loggingEnabled too. Existing users upgrading will get their loggingEnabled re-written as a boolean (already the case) but now via the generic function.
• Windows/Linux tray behavior is untested — isMac guards are in place, but dual-platform behavior should be verified.

Screenshots/Recordings

Dependencies

• No external dependencies. Depends on Electron's built-in Tray and Menu modules.

[kdihalas]

@andrey-dru-me1 i verified this on Linux and works just fine