[Dashboard] Add online/offline status filter for devices

[tsgoff]

Description

This PR adds a status filter to the ESPHome Dashboard device list, allowing users to filter devices by their online/offline status.

Changes Made

• Added three filter buttons to the device list: "All", "Online", and "Offline"
• Filter buttons appear between the search bar and the device grid
• Filter applies only to configured devices (importable/discovered devices are not affected)
• Active filter button is visually highlighted with the primary theme color
• Filter selection persists across page reloads using localStorage

Implementation Details

• Added StatusFilter type: "all" | "online" | "offline"
• New _statusFilter state property to track the current filter selection
• Enhanced _filter() method to check device online status from _onlineStatus
• Filter state is saved to localStorage as esphome-status-filter
• On component initialization, the saved filter is restored from localStorage

Use Case

Users with many ESPHome devices can now quickly:

• View only online devices to check active devices
• View only offline devices to identify devices that need attention
• Toggle between views without losing their search query

Testing

• Built successfully with script/build
• Filter UI renders correctly above device grid
• Filter logic correctly shows/hides devices based on online status
• Filter selection persists after page reload
• Works alongside existing search functionality

[balloob]

We don't want to introduce a new filter bar right now. We rather have #756 land.