feat: Migrate firmware to PlatformIO (ESP32-S3, vesper-v1 env)

Replaces Arduino IDE with PlatformIO as the build system. Entry point
moved from vesper.ino to src/main.cpp. All library dependencies are now
declared in platformio.ini and downloaded automatically via lib_deps.

Board: Kincony KC868-A6 (ESP32-S3, 4MB flash) → env:vesper-v1
Future variants Vesper+ and Vesper Pro are pre-configured but commented out.

Compatibility fixes applied for this framework version:
- Removed ETH.h dependency (Ethernet was disabled in v138)
- Watchdog init updated to IDF v4 API (esp_task_wdt_init signature)
- ETH.linkUp() check removed from OTAManager

Also adds .pio/ to .gitignore and commits the manufacturing plan docs.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -7,4 +7,10 @@ vesper/CLAUDE.md
 vesper/flutter/
 vesper/docs_manual/
 Doxyfile
-vesper/.claude/
+vesper/.claude/
+
+# PlatformIO — build output and downloaded libraries (never commit these)
+vesper/.pio/
+
+# Claude Code memory/session files
+.claude/