Add board photos (1600 px, optimized) and Home Assistant screenshot; integrate into README, photos index, changelog

CHANGELOG.md

@@ -3,6 +3,15 @@
 All notable changes to this project are documented here.
 The format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 
+## [Unreleased]
+
+### Added
+
+- **Photos** (`photos/`): assembled-board photos (front/back, assembled and
+  soldered), an active CO₂ measurement shot, and a Home Assistant dashboard
+  screenshot. Committed to the tree, embedded in the README, and attached as
+  assets to the v1.0.0 release.
+
 ## [1.0.0] - 2026-10-19
 
 First public release of the ESP32-C3 CO₂ & environment sensor board (prototype v1).