feat(eq): reference target curve overlay on the Aetherial Parametric EQ

[ten9876]

Adds a "Ref:" dropdown to the left of the existing "Smoothing:" combo
in both the Channel Strip's EQ panel and the floating ClientEqEditor.
Five selectable presets render as a thin amber line over the EQ canvas
so the user can shape the parametric EQ toward a known target shape:

• AT&T 1959 - Bell Labs "optimum transmission frequency response for
  speech" from the 1959 AT&T handbook. Peak +5 dB at 2.5
  kHz, rolls off below 300 Hz and above 3.4 kHz. The
  canonical presence-peak intelligibility target cited
  throughout broadcast and SSB audio engineering.
• Heil DX - Bob Heil's aggressive DX/contest curve. Sharper +6 dB
  peak at 2.7 kHz, harder low-cut than AT&T 1959.
• Astatic D-104 - Classic crystal "lollipop" mic, very peaky around
  3 kHz with deep low-end rolloff. Digitized from the
  "legendary mics" comparison chart.
• Shure 444 - Broadcast-style desk mic, gentler and broader response.
• Heil HC-5 - Modern dynamic SSB element.

Implementation:

• ClientEqCurveWidget gets setReferenceCurvePreset(QString) and a
  static referenceCurveIds() list (canonical IDs for combo + settings).
  Curves are stored as { hz, dB } point arrays joined point-to-point
  in log-freq x linear-dB space (standard target-curve rendering),
  drawn after the FFT analyzer but before the EQ band curves so the
  user's adjustments sit on top of the target.
• Selection persisted to AppSettings as "ClientEqReferenceCurve"
  (string ID; "Off" hides the overlay). Shared between RX/TX
  editors and between the strip view and the floating editor.
• Toolbar: same QComboBox styling as the existing Smoothing combo,
  placed immediately to the left.

Phase 1 of the request - on/off toggle with a single curve - was
expanded to the 5-preset version after user feedback referencing the
familiar "legendary mics" chart that ham operators compare against.

Files:

• src/gui/ClientEqCurveWidget.{h,cpp} - API + preset tables + paint
• src/gui/ClientEqEditor.{h,cpp} - toolbar combo + persistence
• src/gui/StripEqPanel.{h,cpp} - same toolbar combo for strip view

Co-Authored-By: Claude Opus 4.7 (1M context) noreply@anthropic.com