Document V1/V2 MQTT topics and their code touchpoints in README

[Copilot]

The README didn't explain the V1/V2 protocol topic schemes or where in the codebase they're consumed, making it easy to update one location (e.g. subtopic tables) and miss the others.

Changes

• New "V1 vs V2 Topics" section in README.md, placed before "Authentication"
  • Describes the V2 topic shape (ws-b2d/ws-d2b envelope-based) vs V1 (wprsnpr flat per-message)
  • Points to the three places that must stay in sync when topic/subtopic structure changes:
    • frontend/stores/subscriptions.js — SUBSCRIPTION_MODES topic filters
    • frontend/util.js — TOPIC_MESSAGE_MAP / resolveV1TopicName decoding
    • protobufs.js — resolveV1Topic / decodeByTopic decoding
  • Flags that the frontend and backend V1_SIGNAL_SUBTOPICS tables are intentionally duplicated (frontend can't import the backend module) and must be manually kept in sync

[tyeth]

I expected it to also include hints to users on how to work with an alternative protobuf source and merely that the topics would need adjusting. It's also the case the V1 and V2 documentation of topics and stuff is useful so that's fine too