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/serialio/manager.py

@@ -51,6 +51,7 @@ class SerialManager:
         self._thread: threading.Thread | None = None
         self._stop = threading.Event()
         self._target: _Target | None = None
+        self._open_error = ""
         self.state = State.DISCONNECTED
 
     # ---- public API (any thread) -------------------------------------
@@ -89,9 +90,14 @@ class SerialManager:
         info = find_port(self._target.serial_number, self._target.device)
         if info is None:
             return None
+        posix = sys.platform != "win32"
         # serial_for_url also accepts test URLs such as "loop://".
+        # exclusive: a second tool window must not read the same port (on
+        # macOS/Linux both would get half of the lines). COM ports on Windows
+        # are always exclusive.
         ser = serial.serial_for_url(
-            info.device, baudrate=self._baud, timeout=READ_TIMEOUT_S, do_not_open=True
+            info.device, baudrate=self._baud, timeout=READ_TIMEOUT_S, do_not_open=True,
+            **({"exclusive": True} if posix else {}),
         )
         # On USB-Serial-JTAG, RTS=1/DTR=0 holds the chip in reset and DTR=1
         # during reset selects download mode.
@@ -100,7 +106,6 @@ class SerialManager:
         # macOS/Linux: the OS asserts both lines on open and pyserial then
         # writes DTR before RTS, passing through RTS=1/DTR=0 and resetting
         # the box on every connect. Drop RTS first, then DTR.
-        posix = sys.platform != "win32"
         if not posix:
             ser.dtr = False
         ser.rts = False
@@ -108,8 +113,12 @@ class SerialManager:
             ser.open()
             if posix:
                 ser.dtr = False
-        except (serial.SerialException, OSError):
+        except (serial.SerialException, OSError) as exc:
+            busy = "exclusive" in str(exc).lower() or "busy" in str(exc).lower() or "access" in str(exc).lower()
+            self._open_error = ("The port is used by another program (another Technician Tool window?)"
+                                if busy else "")
             return None
+        self._open_error = ""
         self._target.device = info.device  # COM number / node may change
         return ser
 
@@ -143,7 +152,7 @@ class SerialManager:
                     ser = self._open()
                     if ser is None:
                         if not announced_wait:
-                            self._set_state(State.WAITING, "Plug in / power on the sensorbox")
+                            self._set_state(State.WAITING, self._open_error or "Plug in / power on the sensorbox")
                             announced_wait = True
                         self._drain_cmds(None)
                         time.sleep(REOPEN_POLL_S)