Add opt-in selectedLineFgColor theme option

[0x3654]

Fixes #543
Refs #393, #685

Why

Since #372 (2022) the selected line is a coloured background with the cell's
own foreground force-brightened and emboldened by gocui. Two long-standing
complaints follow from that:

• on dark palettes the default blue background plus bright default text
  washes out — selectedLineBgColor always uses bright colours; default theme is illegible in white-on-black terminals #543 (open since 2024), [FR] Make missing more visible. dark blue on black suboptimal #685 (2025), Change selected background color like lazygit #393 (2022, zero replies);
• the only built-in escape, reverse, inverts the whole row including status
  colours, which turns running's green into an unreadable olive — it trades
  contrast for semantics.

There is no knob for the selected line's foreground: the theme exposes a
background only. gocui's View.SelFgColor exists but is ignored by the main
draw path, so lazydocker could not expose one even if it wanted to.

A default can't fix this — so this PR doesn't change one

ANSI colours are defined by the terminal palette, and palettes disagree:

same config value selectedLineBgColor: [blue]	renders as
Ghostty, Monokai Pro Ristretto (palette 4 = #f38d70)	salmon, readable
classic palettes (ANSI 4 = dark navy)	navy + bright text — the #543 wash-out

Any new default would be a lottery across themes. So the new key is strictly
opt-in: unset by default, and with it unset this PR changes nothing —
rendering is byte-identical to current master. Users who want a uniform,
predictable selection style opt in with one line and pair it with a background
that fits their palette.

What it does

gui:
  theme:
    selectedLineBgColor:
      - blue        # or any colour / hex
    selectedLineFgColor:
      - black       # opt-in; pair with your bg

When set, the whole selected line renders uniformly in that colour:

• plain text keeps the classic bold emphasis;
• cells that carry their own colour (container status, ports, image) are
  overridden, so the row reads as one plate instead of mixed bright colours
  on a coloured background;
• named colours, hex values and attributes work via the existing
  GetGocuiStyle machinery;
• if either key contains reverse, the key is ignored and the classic
  inverted look is kept (an explicit fg under reverse would land on a
  terminal-theme-dependent colour after the swap).

Requires the companion gocui change (honour View.SelFgColor in setRune):
jesseduffield/gocui#107

Screenshots

Six shots, one config (selectedLineBgColor: blue, selectedLineFgColor: black),
two terminals — the palette decides everything.

stock

Terminal.app (classic ANSI blue) — bright text on navy, the #543 wash-out:

Ghostty, Monokai Pro Ristretto — the same blue renders salmon: the palette lottery:

reverse — the existing workaround

Terminal.app — contrast is back, but status colours are inverted into mush:

Ghostty — the same destruction, inverted differently:

this PR, fg: black

Terminal.app — a pair that doesn't fit this palette: pick pairs consciously:

Ghostty — uniform black on salmon: the plate this PR enables:

Same key, same config line — readable or broken depending on the terminal.
That is why this PR adds a knob instead of changing a default.