aethersdr#3396)

## Summary

Two small follow-ups to the applet-slider keyboard support that landed
in aethersdr#3303, plus an audit so the same UX is consistent across every applet
slider. Both features make the keyboard path behave like the mouse path
on applet sliders (AGC level, TCI/DAX gain, etc.).

### 1. Value badge on keyboard steps

When you drag an applet slider with the mouse, a value badge pops up
above the thumb and lingers briefly (~450 ms) after release. Adjusting
the **same slider with the keyboard** previously showed **no badge**.

Now keyboard stepping flashes the identical badge with the identical
linger/fade timeout, so keyboard and mouse give matching visual
feedback.

- `GuardedSlider` (AGC threshold, most applet sliders): gains a public
`flashDragValue()`. Keyboard nudges for these sliders are routed through
`MainWindow`'s shortcut-lease key handler, so that handler calls
`flashDragValue()` after each arrow / `Ctrl`+arrow / `PageUp`/`PageDown`
/ `Home`/`End` step. It's reached via `dynamic_cast` because
`GuardedSlider` has no `Q_OBJECT` macro.
- `MeterSlider` (TCI/DAX combined meter + gain fader): shows and lingers
the badge directly from its own `keyPressEvent`.

### 2. Enter returns focus to the panadapter

After a mouse interaction, a slider holds a short keyboard lease (2 s) /
focus, during which arrow keys nudge the slider instead of driving the
global panadapter shortcuts. If you want those global shortcuts back
**immediately** — without waiting for the lease to expire — you can now
press **Enter** while the slider has focus.

- `GuardedSlider`: `MainWindow` releases the shortcut lease and clears
slider focus, re-enabling the global operating shortcuts.
- `MeterSlider`: hides the badge and clears focus.

### 3. TCI/DAX (MeterSlider) — making the keyboard path actually
reachable

The first pass added the badge + Enter handling to `MeterSlider`, but
that code was **dead**: `MeterSlider` is a plain `QWidget` with
`Qt::TabFocus`, so a mouse drag never left keyboard focus on it, and it
never engaged the shortcut lease. Because the arrow keys are **global
shortcuts** (`Left`/`Right` tune frequency, `Up`/`Down` adjust AF gain),
a matched `QShortcut` consumes the key before the focused widget's
`keyPressEvent` runs — so `MeterSlider`'s own arrow stepping (and the
new badge) never executed.

Fixes, mirroring `GuardedSlider` behavior exactly:
- `MeterSlider`: `Qt::TabFocus` → `Qt::StrongFocus`, plus an explicit
`setFocus(MouseFocusReason)` on left-press, so a mouse interaction hands
off to the keyboard. Adds `isDragging()`.
- `MainWindow`: the shortcut lease is generalized from
`QAbstractSlider*` to `QWidget*` so `MeterSlider` can hold it (freeing
the arrows from the global tune/AF shortcuts while focused).
`focusChanged` and the mouse-press event filter begin the lease for
`MeterSlider` too, and a keypress hook renews it on each step so it
doesn't expire mid-adjustment. A `leaseHolderBusy()` helper unifies
`isSliderDown()`/`isDragging()` so the lease never times out mid-drag
for either type.

### Applet-slider audit (gaps found)

| Slider type | Count | Mouse badge | Keyboard badge | Status |
|---|---|---|---|---|
| `GuardedSlider` | 66 | ✅ | ✅ (this PR) | covered |
| `MeterSlider` (TCI/DAX) | 4 | ✅ | ✅ (this PR) | covered |
| plain `QSlider` | 8 | ❌ | ❌ | pre-existing gap, see below |

The 8 plain `QSlider`s have **never** shown a badge (mouse or keyboard):
`AetherDspWidget` DFNR atten/beta, `StripFinalOutputPanel`
freq/level×2/morse-pitch, `StripWaveformPanel` window,
`SpectrumOverlayMenu` line-width. Most live in transient
**dialogs/menus**, not the main applet panel, and the keyboard-badge
path depends on the panadapter shortcut lease that only applies in the
main window. Converting them to `GuardedSlider` would give them the
mouse badge for free; left out of this PR to keep it focused — easy
follow-up if wanted.

## Behavior summary

| Action | Before | After |
|---|---|---|
| Keyboard-step an applet slider (incl. TCI/DAX) | no badge | badge
flashes + lingers (same as mouse) |
| Press Enter with slider focused | nothing; wait out the lease | global
panadapter shortcuts resume immediately |

## Testing

- Builds clean (Ninja / RelWithDebInfo, Qt6).
- Deployed to the test Mac for hands-on verification of badge timing,
TCI/DAX keyboard stepping, and Enter-to-release behavior.

💻 Generated with Claude Code (Opus 4.8) with architecture by @jensenpat

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Co-authored-by: Codex <noreply@openai.com>