Fullscreen, live link filter toggle & auto-refresh

New Features

• Fullscreen mode — new toggle button in the card header to expand the map to fill the entire screen
• Link filter toggle — switch between parent-child and all-links view on the fly without editing config (initial state from link_filter config)
• Auto-refresh on empty data — when HA restarts and the map sensor is empty, the card automatically requests a refresh once and shows a "Requesting network map data…" banner while waiting

Bug Fixes

• Phantom link crash — filter out links referencing non-existent nodes (broadcast addresses, stale devices) that would crash d3-force with "node not found" error (fixes #2 — thanks @sunlixWhyNotAvailable)
• Invalid HACS repo link — fixed HACS button URL in README.md (#1 — thanks @AndrewPetz)
• Link label opacity — LQI labels now inherit the opacity of their parent link (e.g. neighbor labels render at 0.3 instead of full opacity)
• Click-to-highlight opacity — focused links and labels render fully solid; unfocused elements dim correctly

Improvements

• Refresh button moved to footer — click the timestamp or the refresh icon to trigger a refresh
• Refresh button and warnings hidden when using mock_data
• Tuned default force simulation parameters for better layout with ~20 node networks
• Updated troubleshooting docs (startup automation no longer needed)