feat(theme): selected text colour as a theme field

[kipavy]

Refs #317

Problem

A theme can set the selection background but not the colour of the text inside it. Selected text always keeps its own colours, so a theme cannot reproduce the Termius Dark look from the issue (selected text turns green).

Change

• TerminalTheme takes an optional selectionForeground. It goes to xterm with the rest of the terminal palette. Unset keeps today's behaviour, so existing themes and the built-in presets are unchanged.
• Theme creator: new "Selected Text" row under Terminal Base. Unset shows an empty swatch and a dash. The picker opens on the foreground colour, and once a colour is picked the row gets a reset button.
• terminalColorsFor uses the field for selectionFg, so tmux copy mode follows it. An unparseable value falls back to the foreground.
• Two new strings in all six locales.
• The creator's UI and terminal colour rows were two copies of the same markup. They now share ColorRow and RowIconButton.

Testing

• tsc --noEmit clean, related vitest suites pass (43 tests, 2 new in terminalColors.test.ts).
• Live debug build in a private headless container, WebGL renderer:
  • pick, reset, pick again, save: the saved theme carries the field and becomes active
  • with the field set, red, yellow and bold cyan lines all take the theme colour while selected
  • clearing the field on the active theme restores per-cell colours in an open terminal without a reload
  • the other colour rows and their highlight buttons still work after the refactor

Not covered

• Android was not checked.
• The live run used a debug binary from Oct 9 with the current frontend. The change is frontend-only.
• Terminal rows still pass the picker a 7-character value, so a translucent selection opens as opaque. Unchanged here.