Add FLARM aircraft display support

[lefrogfrog]

Adds support for displaying FLARM aircraft from OpenSky's Avro-deserialised FLARM data alongside existing ADS-B traffic in the tar1090 frontend. See FLARM.md for full implementation details.

Changes:

• html/script.js — Added normalizeFlarmAircraft() converter (JSON fields -> tar1090-compatible fields), processFlarmUpdate() fetch/poll loop wired via setIntervalTimers(), refreshSelectedFlarm() sidebar renderer, FLARM legend entry, FLARM source filter button, and Non-ICAO info block guard.
• html/planeObject.js — FLARM classification in updateData() (sets dataSource = "flarm", preserves raw Avro fields under this.flarm), setNull()/planeCloneState() updates.
• html/index.html — FLARM detail block (hidden by default, shown on selection) inside infoblock-container.
• html/formatter.js — case 'flarm': return "FLARM" in source label formatter.
• html/markers.js — FLARM aircraft render with the glider SVG icon via early return in getBaseMarker().
• html/defaults.js — flarm_server / flarm_refresh config variables.
• html/config.js — Commented example overrides.
• html/data/flarm_aircraft.json — Example mock data file.
• FLARM.md — Implementation documentation.
  Key design decisions
• FLARM aircraft use synthetic hex keys (~F) to prevent namespace collision with ADS-B in g.planes.
• Source classification (dataSource = "flarm") is independent of ADS-B category codes.
• Raw Avro fields preserved in a flarm sub-object; only position/velocity/rate fields are mapped to tar1090 top-level equivalents (lat/lon, alt_geom, gs, geom_rate, track, track_rate).
• Filter toggle, legend colour, marker icon, and sidebar detail section all distinguish FLARM from ADS-B.