Settings: background and accent colours under Font size, PT Default first and default, a blurred help popup

[Maxaubert]

Stacked on #39 (merge that first; this PR's base is its branch).

For the owner

You asked for:

• the help popup without its shadow, with a blurred background;
• an accent colour option that picks "the accents you see like the blue highlight effect and tab effect";
• a background colour setting;
• both moved "to the top of the list right under font and font size";
• your palette as the default theme for Prism Terminal, first in the list, called PT Default.

What changed

• Settings > Appearance > Background colour and Accent colour, directly under Font size. Each shows the theme's own colour until you pick one; then a plain Reset word (Prism's style, from Settings: picking a theme moves nothing, and Reset is a word, as in Prism #39) puts it back.
• The background repaints the whole window, terminal included. Text, dim text and the accent are re-measured against it, so they stay readable. A light colour turns the window light.
• The accent repaints the active tab's line, the selected row and settings page, pressed buttons and primary buttons. A colour you pick is kept as it is, and only moved when the background cannot show it.
• PT Default is the default theme, first in the wall: your saved palette (Wombat's colours on #121212) with #FE8F34 as its accent. Anyone who never picked a theme moves to it with this update; anyone who picked one keeps theirs. The wall now leads with the app's own default (Prism's default is "follow style", so its wall is unchanged; PT Default is just one more preset there).
• The help popup blurs the window behind it and casts no shadow (in Prism too, since it is the core's).
• Core 0.8.0 (the new preset, and a slot so the app's own rows can sit under Font size). App 0.10.0.

Not done, say if you want them

• The agent working indicator still follows the theme's accent, not the one you pick. Changing it is a core change and a host option, so it's your call.
• With a picked background, the terminal's ANSI colours are still measured for readability against the theme's own background. The same goes for the character under the block cursor. Both live in the core. They only matter if the picked background is far from the theme's.

Tests

• helpPanel asserts the blurred background (blur(6px)) and no shadow.
• typecheck clean, lint 0 errors, unit 843 passed (new: backgroundPrefs.test.ts, a light background turning the window light with readable inks, and the accent's kept/floored rules).
• Full e2e: all scenarios passed. The accent scenario now also covers:
  • a fresh profile wears PT Default, and it is the first card;
  • Background and Accent sit right after Font size;
  • a picked background becomes the window's ground, and Reset brings the theme's back;
  • the accent reaches the active tab's line and a pressed button.

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