feat: rework the tablet about tab, and theme both tablet diagrams

[TheSevenPens]

Three passes over the tablet about and mapping tabs. They grew on one branch as one thing led to the next; they touch different files and can be read independently.

1 · The active area, drawn

┌ ─ ─ ─ ─ ─ ─ ─ ╱ ┐
│           ╱     │
│   268.5 mm      │  148 mm
│ ╱               │
└ ─ ─ ─ ─ ─ ─ ─ ─ ┘
      224 mm

The numbers were already in the rows above it. What the drawing adds is the shape — how wide or how square a tablet is reads instantly from a rectangle and not at all from "224 × 148 mm". Scaled on its longer edge to a fixed 232 DIP box, so a wide tablet and a squarer one are drawn at comparable visual weight. The diagonal runs lower-left to upper-right, dashed so it reads as a measurement across the area rather than an edge of it.

BASICS becomes MODEL and keeps only the tablet's identity; the three size rows move to a new ACTIVE AREA section beside the drawing. Their labels shorten from Active area / Active area diagonal / Active area aspect ratio to Size / Diagonal / Aspect ratio — the prefix carried context under BASICS and merely repeated the heading under ACTIVE AREA.

2 · Section headings that stop out-typing the tab

Those headings started as SectionLabel largetitle — 13px uppercase SemiBold in muted ink — which is a different answer to the same question than Home's, in one app. Home's headings are 44px Inter Light lowercase in primary ink, and that is what the redesign is built around, so these moved to match.

At 44 they were larger than the tab menu containing them: about in the tab row is TypePivotSize (40), so model beneath it read as the bigger deal. Home has no such problem — its page menu is TypeNavSize (22), giving Display a clean 2× jump.

New TypeSectionSize (22) and a TextBlock.Display.section modifier put them a rung down: 40 tab menu → 22 section → 13 body, same voice, no longer competing. The token equals TypeNavSize today but is deliberately its own: retuning the page menu should not drag section headings with it. LetterSpacing goes -1.4 → -0.7, since -1.4 was drawn for a 44px heading and sets the letters twice as tight at half that size.

3 · The mapping diagram, drawn from the theme

The diagram was half themed and half hardcoded. Beams and the active-area outline read AccentBrush; the selected display was nailed to #6366F1 — which is the Light theme's accent. On any other skin the beams changed colour and the box they pointed at did not, so pink beams arrived at an indigo screen. A hardcoded #22C55E bottom beam and cool fixed greys (#D9D9E3 display, #8A8A92 tablet) put five hue families on a warm pink ground.

Now one hue. The accent marks only the mapping — selected border, active-area outline, all three beams — and every other surface is ink at a low alpha, so neutrals tint themselves to whatever ground they sit on. Selection is carried by the border, the glow and a slightly heavier fill rather than a saturated block.

New InkBrush on both diagrams, bound to TextPrimaryBrush.

This reaches past the mapping tab. DiagramDrawing's TabletFill/TabletBorder are shared by both diagrams on purpose (#620), so the same tablet does not read one way on Active Area and another on Display Mapping. Retinting one required retinting both — the Active Area tab's tablet moves with it.

Two things only the running app showed:

• Beam alpha 0x9C → 0x60. Legible against saturated boxes; against tinted neutrals the beams became the loudest thing in the diagram, and where the three overlap the alpha compounded into a solid wedge.
• ActiveAreaDiagram's "No active-area data" was Brushes.White — fine on a #8A8A92 fill, invisible against the light tint replacing it. Now ink at 0xA6.

For the reviewer

• The About-tab diagonal is built as a Geometry in the view model and bound straight to Path.Data. The obvious alternative — a nested LineGeometry with its EndPoint bound to a Point — relies on a non-visual element inheriting DataContext, which renders blank without erroring.
• Verified in both tablet states: drawn to scale for a connected PTH-660, and ACTIVE AREA hidden entirely for a remembered tablet with no dimensions, MODEL still carrying the "Connect this tablet…" line and no empty box left behind.
• Themes: checked by hand in Sakura and Dark. Light is not hand-checked, but every colour now derives from AccentBrush and TextPrimaryBrush rather than from a literal.
• Drops a dead Muted brush in ScreenMappingDiagram — declared, never referenced.
• 673/673 locally. First CI run on the new Avalonia.Media use in TabletDetailViewModel, so macOS and Linux see it here for the first time.