Serial monitor redesign: Test box button, InfluxDB row in the checklist

- Test box: restart + start-up capture + online InfluxDB check (data newer
  than the restart), with a status strip; checklist and findings are the
  main view, live log / guided troubleshooting / box details are tabs,
  rarely used actions in a More menu
- light palette on all systems (macOS dark mode made the colours unreadable)
- new rule influx.invalid_value: -inf from a silent microphone makes
  InfluxDB reject every upload (HTTP 400); guide end names it
- exclusive serial port on macOS/Linux: two tool windows no longer split
  the log between them
- InfluxDB check: `since` filter, neutral wording when nothing new arrives

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

kota_tech/__main__.py

@@ -1,13 +1,42 @@
 import sys
 
+from PySide6.QtGui import QColor, QPalette
 from PySide6.QtWidgets import QApplication
 
 from .ui.main_window import MainWindow
 
 
+def light_palette() -> QPalette:
+    """The same light look on every system. All status colours in the tool are
+    chosen for a light background; with macOS dark mode they were unreadable."""
+    p = QPalette()
+    roles = {
+        QPalette.ColorRole.Window: "#f4f5f7",
+        QPalette.ColorRole.WindowText: "#1d1d1f",
+        QPalette.ColorRole.Base: "#ffffff",
+        QPalette.ColorRole.AlternateBase: "#f0f2f5",
+        QPalette.ColorRole.Text: "#1d1d1f",
+        QPalette.ColorRole.Button: "#e9ebef",
+        QPalette.ColorRole.ButtonText: "#1d1d1f",
+        QPalette.ColorRole.ToolTipBase: "#ffffe8",
+        QPalette.ColorRole.ToolTipText: "#1d1d1f",
+        QPalette.ColorRole.PlaceholderText: "#8a8a8a",
+        QPalette.ColorRole.Highlight: "#2e6da4",
+        QPalette.ColorRole.HighlightedText: "#ffffff",
+        QPalette.ColorRole.Link: "#1f5fa8",
+        QPalette.ColorRole.BrightText: "#ffffff",
+    }
+    for role, colour in roles.items():
+        p.setColor(role, QColor(colour))
+    for role in (QPalette.ColorRole.WindowText, QPalette.ColorRole.Text, QPalette.ColorRole.ButtonText):
+        p.setColor(QPalette.ColorGroup.Disabled, role, QColor("#9a9a9a"))
+    return p
+
+
 def main() -> int:
     app = QApplication(sys.argv)
     app.setStyle("Fusion")
+    app.setPalette(light_palette())
     win = MainWindow()
     win.show()
     return app.exec()