… Principle XI. (#3839)

## The bug

The frequency readout **clips its leading (most-significant) digits** in
two places — the VFO slice flag (`VfoWidget`) and the RX Controls box
(`RxApplet`). The operator loses the MHz digits, the worst ones to lose.
Reported in #3463 (Default Light, Windows/FLEX‑6400, "first number
cut"); also visible on macOS default **dark** in the RX Controls box.

## Root cause

Both surfaces render the frequency as a plain `QLabel` styled by the
theme tokens `font.size.freq` / `font.family.freq` (DSEG7 Modern, 20px
dark / 26px light), poured into a **fixed-width box with no
shrink-to-fit and no elision**. When the rendered DSEG7 string is wider
than the box, the right-aligned label clips on the left:

- **`RxApplet`** — the freq stack shares its row with TX/mode/WFM inside
the fixed **~248px** `AppletPanel` column. The leftover is **~124px**
(measured live via the bridge), narrower than even `21.074.000` at the
default 20px → the leading **`2`** clips **on the default dark theme**.
In light theme (26px) only `074.000` survives.
- **`VfoWidget`** — the 252px flag fits 2-digit-MHz HF in dark, but the
light-theme 26px digits (and the wider Windows metrics in #3463)
overflow and clip the leading digit.

This is the unfinished half of #844/#851 (which widened the digit
*template* for 4-digit MHz but left a hardcoded pixel probe), and it
mirrors the font-derived-sizing precedent from #3503.

## The fix

A QSS `font-size` **overrides** `setFont()` (verified empirically),
which is why the size couldn't be adjusted in code before. So:

- Drop `font-size` from each freq label's stylesheet
(family/weight/colour stay, theme-reactive) and drive the pixel size via
**`setFont()` with a width-fitted size**, capped at the theme nominal —
new shared `src/gui/FreqLabelFit.h`.
- Re-fit on text update, label `QEvent::Resize`, and
`ThemeManager::themeChanged` (so Dark↔Light and Theme-Editor font edits
stay correct).
- `RxApplet`'s freq stack is made `Expanding` (stretch removed) so it
fills a **font-independent** width — avoids a fit↔reflow oscillation.

Digits stay **full-size whenever they fit** and shrink only as much as
needed to keep the leading digits on screen. (This fixes the clipping;
making the digits *larger* — #3515 — remains a separate enhancement.)

## Proof — agent automation bridge

Driven against a live FLEX‑8400M (RX-only, `tune` + `grab`), before vs
after, both themes. Top row = before (clipped), bottom row = after
(fixed):

**RX Controls box — default dark, 21.074 MHz** (leading `2` clipped →
full)
![RxApplet dark
before/after](https://raw.githubusercontent.com/jensenpat/AetherSDR/assets-vfo-freq-clip/pr-rxapplet-dark.png)

**RX Controls box — light theme, 21.074 MHz** (only `074.000` visible →
full)
![RxApplet light
before/after](https://raw.githubusercontent.com/jensenpat/AetherSDR/assets-vfo-freq-clip/pr-rxapplet-light.png)

**VFO slice flag — light theme, 21.074 MHz** (leading `2` clipped →
full)
![VfoWidget light
before/after](https://raw.githubusercontent.com/jensenpat/AetherSDR/assets-vfo-freq-clip/pr-vfo-light.png)

| Surface / case | Before | After |
|---|---|---|
| RxApplet, dark, 21.074 MHz | leading `2` clipped | full `21.074.000` |
| RxApplet, dark, 144.2 MHz | `1` gone, `4` clipped (`~44.200.000`) |
full `144.200.000` |
| RxApplet, light, 21.074 MHz | only `074.000` | full `21.074.000` |
| VfoWidget, light, 21.074 MHz | leading `2` clipped | full `21.074.000`
|
| VfoWidget, dark (already fit) | — | unchanged, no regression |

## Files

- `src/gui/FreqLabelFit.h` (new) — `fitFreqPixelSize()` +
`applyFittedFreqFont()`
- `src/gui/VfoWidget.{h,cpp}`, `src/gui/RxApplet.{h,cpp}` — drop QSS
font-size, `applyFreqFit()`, re-fit hooks, RxApplet stack `Expanding`

Closes #3463. Refs #3515.

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

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>