…Sb (aethersdr#2180)

Both setters updated m_micLevel / m_monGainSb and emitted commandReady
but never emitted micStateChanged(), so any UI control bound to mic
state via the model signal — e.g. a MIC level slider updated by MIDI
controller, keyboard shortcut, or radio echo — would not repaint until
the next unrelated state event.

The fix mirrors the pattern established for VOX setters in aethersdr#2084
(setVoxEnable/Level/Delay all emit phoneStateChanged() unconditionally).

Full setter audit: all other setters that mutate mic-state members
(setMicSelection, setSpeechProcessorEnable, setSpeechProcessorLevel)
already emit micStateChanged(). setMonGainCw and setMonPanCw correctly
emit phoneStateChanged().

Fixes aethersdr#2180

## Problem

`TransmitModel::setMonGainSb()` updates `m_monGainSb` and emits
`commandReady` but never emits `micStateChanged()`. A UI slider bound to
monitor gain via the model signal won't repaint when the value is
changed by MIDI controller, keyboard shortcut, or radio echo — until an
unrelated event happens to trigger a redraw.

Same class of gap as #2084 (VOX setters) and the `setMicLevel` part of
#2180.

**Note:** `setMicLevel` was fixed concurrently in PR #3234 (with a
change guard). This PR addresses the remaining gap — `setMonGainSb`.

## Fix

Add `emit micStateChanged()` after updating `m_monGainSb`, consistent
with how all other mic-state setters behave.

## Setter audit (per #2180)

| Setter | Signal emitted? |
|---|---|
| `setMicSelection` | ✅ `micStateChanged()` |
| `setMicLevel` | ✅ `micStateChanged()` — fixed in #3234 |
| `setSpeechProcessorEnable` | ✅ `micStateChanged()` |
| `setSpeechProcessorLevel` | ✅ `micStateChanged()` |
| `setMonGainSb` | ❌ **missing** — fixed here |
| `setMonGainCw` | ✅ `phoneStateChanged()` |
| `setMonPanCw` | ✅ `phoneStateChanged()` |

Fixes #2180

…) (aethersdr#3240)

## Problem

`TransmitModel::setMonGainSb()` updates `m_monGainSb` and emits
`commandReady` but never emits `micStateChanged()`. A UI slider bound to
monitor gain via the model signal won't repaint when the value is
changed by MIDI controller, keyboard shortcut, or radio echo — until an
unrelated event happens to trigger a redraw.

Same class of gap as aethersdr#2084 (VOX setters) and the `setMicLevel` part of
aethersdr#2180.

**Note:** `setMicLevel` was fixed concurrently in PR aethersdr#3234 (with a
change guard). This PR addresses the remaining gap — `setMonGainSb`.

## Fix

Add `emit micStateChanged()` after updating `m_monGainSb`, consistent
with how all other mic-state setters behave.

## Setter audit (per aethersdr#2180)

| Setter | Signal emitted? |
|---|---|
| `setMicSelection` | ✅ `micStateChanged()` |
| `setMicLevel` | ✅ `micStateChanged()` — fixed in aethersdr#3234 |
| `setSpeechProcessorEnable` | ✅ `micStateChanged()` |
| `setSpeechProcessorLevel` | ✅ `micStateChanged()` |
| `setMonGainSb` | ❌ **missing** — fixed here |
| `setMonGainCw` | ✅ `phoneStateChanged()` |
| `setMonPanCw` | ✅ `phoneStateChanged()` |

Fixes aethersdr#2180