refactor(ui): Icon-Faerbung zentral im ICON-PAINTING-Block

Inline Foreground/Stroke an Icons entfernt — ein lokaler Wert schlaegt jeden
Style, dadurch erreichten Hover/Checked/Disabled das Icon nicht. Farbe wird
jetzt per Klasse gewaehlt (.danger, .star-btn.on, .active), alle Zustaende
liegen in einem geordneten Block in IslandStyles.axaml.

- Path.plan-icon -> Path.icon-stroke (gilt jetzt fuer Button und ToggleButton)
- Icon.Activity als gefuellte Kontur (PathIcon fuellt, Stroke-Polyline gab Blobs)
- BloodBrightBrush als Hover-Ton fuer destruktive Icons
- icon-btn:disabled bleibt transparent statt Fluent-Grau

src/ClaudeDo.Ui/CLAUDE.md

@@ -113,7 +113,8 @@ snaps `CanResize="True"` windows, which is the opt-in), and it insets itself by
 
 ## Gotchas
 
-- **`PathIcon` *fills* its geometry.** Line-art/stroke icons must be authored as filled geometry or rendered with a stroked `Path` (e.g. `Icon.PlanDay` via the `Path.plan-icon` style). A pure stroke path in a `PathIcon` is **invisible**.
+- **Never set `Foreground`/`Stroke` on an icon inline.** A local value outranks every style, so the button's hover/checked/disabled states can't reach the icon and it looks dead on hover — this is what broke the icons that were "fixed" to the right colour inline. Colour is picked with a class; the single `ICON PAINTING` block in `IslandStyles.axaml` owns all states (`.danger`, `.star-btn.on`, `.active`, `:checked`, `:disabled`). Avalonia has no specificity — that block's **declaration order** is the mechanism, so add new variants in the right section, not at the end.
+- **`PathIcon` *fills* its geometry.** Line-art/stroke icons must be authored as filled geometry or rendered as `<Path Classes="icon-stroke">` inside a `Viewbox` (e.g. `Icon.PlanDay`, `Icon.GitBranch`). A pure stroke path in a `PathIcon` is **invisible**. `Path.icon-stroke` takes the same colour classes/states as `PathIcon`.
 - **`NumericUpDown.Value` is `decimal?` and goes null while the box is empty** — i.e. every time the user clears a value to type a new one. Bound TwoWay to a non-nullable `int`/`decimal`, that null throws `InvalidCastException`. Either bind a `decimal?` property (as `AgentConfigEditorViewModel.MaxTurns` does) or add `Converter={StaticResource KeepLastNumber}`, which drops the null via `BindingOperations.DoNothing`.
 - **`ScrollViewer.Padding` is excluded from the Extent (Avalonia 12).** At max scroll the content still sticks out below the viewport by the padding height, so the last line(s) are clipped and unreachable — even after `ScrollToEnd()`. Put the inset on the *content* (`Margin` on the inner `ItemsControl`/`StackPanel`) instead. Verified headlessly: 12,8,12,4 on the viewer → last item 20px below the viewport; same inset as content margin → fully visible. **`ListBox` does *not* have this trap** — its Fluent control-theme template forwards `Padding` onto `PART_ItemsPresenter`'s margin (content side), not the raw `ScrollViewer.Padding`; verified headlessly that `ListBox.Padding="10,4"` and a `Margin="10,4"` on the `ItemsPanelTemplate` panel produce an *identical* Extent and leave the last row fully visible at max scroll. Still fine to put the inset on the panel for consistency with the other scrollables above, just don't assume it's fixing a real bug there.
 - **Re-templating a `TabControl` must keep `PART_ItemsPresenter`.** `TabControl.SelectedContent` is resolved via `ContainerFromIndex`, and containers are only realized by the `ItemsPresenter`. A template that renders just `PART_SelectedContentHost` (to hide the tab strip) leaves `SelectedContent` permanently `null` → an empty body. Keep the presenter with `IsVisible="False"`; that still realizes containers (verified headlessly, Avalonia 12.0.4). This broke the whole Settings modal body in 56c3719e.