docs: add Weather Predictor design spec

Autonomous barometric weather station on Wemos D1 mini with BMP180,
DS3231 RTC and 0.96" ST7735 TFT. Local Zambretti forecast, web UI,
LittleFS storage, WiFiManager + NTP.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,12 @@
+# Arduino / build artifacts
+build/
+*.bin
+*.elf
+*.hex
+.vscode/
+# OS
+Thumbs.db
+Desktop.ini
+.DS_Store
+# Secrets (Wi-Fi creds are set via WiFiManager, not committed)
+secrets.h