Add bidirectional ESP-NOW API

[figamore]

This PR adds a bidirectional ESP-NOW API for remote controllers while preserving the existing WiZ remote and WLED-to-WLED ESP-NOW sync behavior.

Linked remotes can leverage this API to:

• Send WLED JSON state commands with /json/state-compatible behavior
• Request full or compact state responses
• Receive state updates when WLED changes
• Discover multiple WLED instances and identify their capabilities
• Request effect, palette, and preset catalogs
• Subscribe to binary live LED preview frames

A compatible remote implementation is available at:
https://github.com/figamore/wled-touch-remote

Implementation

• Adds a versioned ESP-NOW wire protocol with request, response, push, discovery, announce, and live-preview message types.
• Fragments and reassembles messages larger than ESP-NOW's 250-byte frame limit.
• Processes ESP-NOW callbacks through bounded queues so JSON parsing, filesystem access, and state changes remain in the main loop.
• Uses the existing deserializeState() path for parity with the HTTP and WebSocket JSON APIs.
• Adds compact state polling and unsolicited state pushes for lower-overhead remote synchronization.
• Adds retry deduplication so repeated mutation requests do not apply non-idempotent changes twice.
• Supports discovery of multiple WLED instances using their link-layer MAC addresses.
• Keeps the fallback AP and ESP-NOW channel stable while an API remote is active.

Pairing UI

The ESP-NOW section of the WiFi settings page has been enhanced:

• Recently detected ESP-NOW remotes, including type (Wizmote or bidirectional ESP-NOW) and last-seen time
• Add/remove controls for linked remotes
• Manual MAC-address entry with validation
• Clear guidance that pairing changes require Save & Connect

Firmware-side validation also rejects malformed and duplicate MAC entries.

Compatibility and limitations

• Existing Wizmote support and WLED-to-WLED ESP-NOW sync remain supported.
• Full verbose responses are limited to 2048 bytes on ESP8266 and 8192 bytes on ESP32. Compact polling remains available when a full response is too large.
• State pushes and live-preview frames are best-effort.
• Only one live-preview subscriber is served at a time.
• Live preview is limited in resolution and frame rate to avoid starving control traffic.

The complete protocol and remote implementation guidance are documented in docs/espnow-json-protocol.md.

Tests performed

• npm test
• Web UI build: npm run build
• ESP32 firmware build
• ESP8266 firmware build
• Tested state requests and responses on hardware
• Tested unsolicited state updates
• Tested discovery and multi-device pairing
• Tested live LED preview
• Confirmed existing WiZ Mote behavior
• Tested STA and fallback AP operation
• Tested across host/remote reconnects

Documentation

• Adds docs/espnow-json-protocol.md covering framing, fragmentation, discovery, catalogs, reliability, security assumptions, limits, and example messages.

Summary by CodeRabbit

• New Features

  • Added bidirectional ESP-NOW control for supported ESP32 devices, including JSON state requests, responses, discovery, live LED previews, and state updates.
  • Added catalog queries for effects, palettes, and playlists.
  • Improved ESP-NOW synchronization and connection handling across Wi-Fi and access-point modes.

• Bug Fixes

  • Strengthened packet validation, sender authorization, duplicate handling, and linked-remote MAC validation.

• Documentation

  • Added comprehensive ESP-NOW JSON protocol documentation.