…ault mismatch

## Problem

Users report that configuring DSR (or CTS) as PTT Input in Radio Setup → Serial
has no effect — pressing the footswitch produces no RF output and no interlock
state change, even when hardware is confirmed working via FRStack.

Three independent bugs blocked the PTT path:

---

### Bug 1 — No way to open the serial port without restarting

`loadSettings()` only opened the port when `SerialAutoOpen == "True"`. The
Serial tab had no Open/Close buttons, so users who configured settings in a
running session had no mechanism to activate the port until the next restart.
Even with auto-open enabled, the label "Auto-open serial port on **startup**"
implied a restart was required — many users did not realize that closing the
dialog alone was sufficient to trigger `loadSettings()`.

### Bug 2 — Polarity default mismatch (silent inversion)

`loadSettings()` fell back to `"ActiveLow"` for `SerialCtsPolarity` and
`SerialDsrPolarity` when the key was absent from AppSettings. The polarity
combo in the dialog defaulted to displaying `"Active High"` (index 0), but its
`currentIndexChanged` signal is connected *after* the initial index is set, so
leaving polarity at the displayed default wrote nothing to AppSettings. Result:
a user who configured DSR → PTT Input and left polarity at the displayed
"Active High" default would have the controller silently invert the logic —
footswitch press reads as inactive, PTT never fires.

### Bug 3 — `updatePolling()` not called when function assignments change on an open port

When the dialog was closed and `loadSettings()` ran with `SerialAutoOpen ==
"False"` (port not being re-opened), the poll timer state was never refreshed.
If a user changed a pin function from None → PttInput while the port was already
open, the 10 ms poll timer would not start and the new assignment would be
silently inert.

---

## Fix

### `src/core/SerialPortController.cpp`

- **`loadSettings()`**: open condition now checks `SerialAutoOpen || SerialPortOpen`
  so the new Open button in the dialog takes effect when the dialog closes.
- **`loadSettings()`**: added explicit close path (`!shouldOpen && isOpen()`) so
  the Close button also takes effect without a restart.
- **`loadSettings()`**: added `updatePolling()` in the else branch (port state
  unchanged) so pin function changes on an already-open port correctly start or
  stop the 10 ms poll timer.
- **`loadSettings()`**: changed `SerialCtsPolarity` and `SerialDsrPolarity`
  fallback defaults from `"ActiveLow"` to `"ActiveHigh"` to match the combo's
  displayed default, eliminating the silent polarity inversion for first-time
  users who leave the polarity combo untouched.
- **`saveSettings()`**: now also persists `SerialPortOpen` so a port opened
  manually via the button survives restart (same semantics as `SerialAutoOpen`).

### `src/gui/RadioSetupDialog.cpp`

- Added **Open** and **Close** buttons to the Serial tab with a live status
  indicator ("Open" green / "Closed" grey). Clicking Open sets
  `SerialPortOpen = True` in AppSettings; clicking Close sets it to `False`.
  Both take effect when the dialog is closed, triggering `loadSettings()` on
  the worker thread via the existing `QMetaObject::invokeMethod` in MainWindow.
- Retained the "Auto-open serial port on startup" checkbox beneath the buttons.

---

## Signal path (verified end-to-end)

```
footswitch asserts DSR (active high)
  → SerialPortController::pollInputPins() [10 ms QTimer, worker thread]
    → dsrActive=true, m_lastDsrActive=false, debounceOk=true
      → emit externalPttChanged(true)           [queued → main thread]
        → RadioModel::setTransmit(true)
          → sendCmd("xmit 1")                   [TCP → FLEX-8600 → RF output]
```

CTS PTT path is identical (`m_ctsFn == InputFunction::PttInput`).

---

## Steps to reproduce (before fix)

1. Connect USB-serial adapter; footswitch on DSR pin, active high.
2. Open Radio Setup → Serial.
3. Select port, set DSR = PTT Input, Polarity = Active High.
4. Close dialog (auto-open not checked, no restart).
5. Press footswitch → no keying, no interlock change.

## Steps to verify (after fix)

1. Same hardware setup.
2. Open Radio Setup → Serial; configure port and pin assignment.
3. Click **Open** → status label changes to "Open" (green).
4. Close dialog.
5. Press footswitch → radio keys, RF output observed.
6. Re-open dialog, click **Close** → status "Closed"; footswitch no longer keys.
7. Enable **Auto-open serial port on startup**, exit and restart → port opens
   automatically on startup, footswitch keys without manual Open step.

---

## Files changed

  src/core/SerialPortController.cpp   loadSettings: SerialPortOpen flag, close path,
                                      updatePolling, polarity defaults (ActiveLow→ActiveHigh)
                                      saveSettings: persist SerialPortOpen
  src/gui/RadioSetupDialog.cpp        Serial tab: add Open/Close buttons + status label