fix(a11y): contrast and focus in every theme

[hazeliscoding]

The second v1.1.1 item from the 2026-10-10 accessibility audit.

Focus

• What changed: a FocusOutline token, an inset accent ring, now shows keyboard focus. It replaces Fluent's square white outline, which sat on top of each control's own focus border.
• Where: buttons, list rows, tabs, segments, Quick save's tiles, and the prompt's title and description, which used to show focus only as a faint fill.
• The focused list row wears the hover fill and the ring, so it no longer looks like a picked one.
• The Switch draws its edge in Text1 instead, since an accent ring would vanish on its accent track.
• Found while building: Avalonia 12.1.3 draws no box shadow on a border whose thickness differs by side. So the focused row's border goes all round, and tabs carry the ring on an outer border.

Contrast

• Control edges: a new ControlBorderBrush reaches 3:1 on every background. It's the faintest step from Border3 toward Text3 that does. Text boxes, dropdowns, the library's checkboxes and the editor's empty rating pips use it.
• Tags and danger buttons reach 4.5:1 on their own tint.
  • Tone text is nudged in eight palettes: Darcula, Gruvbox Dark, Dracula, Nord, Solarized Dark, Prompuff Light (mint), Solarized Light and Catppuccin Latte.
  • Danger buttons sat below 4.5:1 even at rest in Gruvbox Dark, Nord and Dracula.
• Danger hover shows in the button's edge. It used to be white on solid red, at 2.3–3.4:1.
• Counts on the active sidebar row use Text2. Text2 is nudged in Darcula, Gruvbox Dark and Solarized Dark to reach 4.5:1 on Bg4.
• The picked segment (Cards/List, theme, density, release channel, Changes/Full text, MCP client) gets the accent's fill and edge with Text1. It used to sit at 1.01:1 in Prompuff Light.

Tests

• New ThemeTests case: for all thirteen themes, it reads each pair from the resources that theme actually resolves.
  • Control edges and the accent at 3:1 on Bg0–Bg3 and the inset.
  • Each tone's text on its own tint over Bg1–Bg3.
  • Text2 on Bg4.
  • Text1 on the accent tint.
• The token check that compares Prompuff's own themes with Tokens.axaml now covers ControlBorderBrush and the mint text too.
• Looked at: screenshots of the library in cards and list view, the editor, a dialog, Settings and Prompuff Light.