Accessibility: color and channel sliders report their values to screen readers

The color and channel sliders gave screen readers a name but no value.
Each caller maps its own range onto the thumb's position, so a screen
reader announced the Color panel's G slider, Transform Each's Scale or
the Navigator's Zoom without the setting the field beside it shows.

color_slider_valued and channel_slider_valued take an optional
SliderValue: the number and text of the field beside the slider and
the slider's range. The accessibility node gets the number, the text
and the range, which extends to a number past the slider's end.
color_slider_named and channel_slider_named keep their arguments and
report no value. Every caller outside the tests uses the new variants:
the Color panel, Swatch Options, the Color Picker, the Navigator, the
Character panel's variable font axes and the dialog sliders.
form::slider_rail_named reports the number it edits, and the harmony
wheel's Brightness slider, which has no field, reports its brightness
in percent.

Docs: docs/target-app-parity.md.

docs/target-app-parity.md

@@ -457,6 +457,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
   - With anchor points selected with Direct Selection, the contextual task bar offers Remove Anchor Points, Connect (`path.join`), Cut Path, Corner and Smooth (`path.convertAnchors` with `to` set to `corner` or `smooth`), the commands of the Control bar's anchor buttons. As in Illustrator ("Remove selected anchor points, Connect selected end points, Cut path at selected anchor points, Convert selected anchor points to corner, and Convert selected anchor points to smooth": [Contextual Task Bar overview](https://helpx.adobe.com/illustrator/desktop/get-started/learn-the-basics/contextual-task-bar-overview.html)). Not done: Simplify, which the bar offers only while no anchors are selected, and the bar's Smooth item that adjusts a path's curvature, a command the app doesn't have (Object › Path › Smooth).
   - Tool flyouts tear off: dragging or clicking the tear-off bar on a flyout's right side (or releasing the long press over it) floats its group as a strip of tool buttons, moved by the same bar and put back in the toolbar by the × at its top (`window.floatTools`). While a group floats, the presses that would open its flyout raise the strip instead. Floating strips are kept with the preferences and in user workspaces. Not done: turning a strip vertical, docking it.
   - Screen readers get a name for the Tools panel's tool buttons, flyout rows, floating strips and tear-off bars, and for the icon buttons, text buttons, toggles, checkboxes, radio buttons, tabs, chips, sliders, angle dials and combo boxes that panels and dialogs draw with `crates/ui-egui/src/widgets.rs`: the text, label or tool tip each one shows, or a name of its own where it shows none (the angle dial, the spinner, the preset arrow, the color wheel and the Reference Point's squares). The tool buttons, toggles, checkboxes, radio buttons, tabs and the Reference Point's nine squares also report whether they are on, an icon button that is lit, such as the chosen Align To button, reports that it is pressed, and the angle dial reports its angle. The status bar's First, Previous, Next and Last Artboard buttons have tool tips.
+  - Screen readers get the value of the color and channel sliders that panels and dialogs draw (the Color panel, the Color Picker, Swatch Options, the Navigator's Zoom, the Character panel's variable font axes, the Perspective Grid and flattener options, and the sliders of the effect, tool option and transform dialogs): the number and text of the field beside each one, such as 128 for G or 50% for an opacity, with the slider's range, which extends to include a number past the slider's end. The harmony wheel's Brightness slider, which has no field, reports its brightness in percent.
   - Panels undock (#495): a dock tab, a panel icon or a popped-out panel's title dragged out of the dock floats that panel inside the window, and the strip right of the dock's tabs floats the whole tabbed group. A floating group moves by its title bar, a tab dragged out of it floats on its own, and dropped on another group's title bar or tabs it stacks with that group; dropped on the dock, which lights up under the pointer, its panels go back where they live (the tabbed group or the icon column), as its × and the panel menus' Float Panel / Dock Panel do (`window.panel.float`, `window.panel.dock`). The Tools panel floats by its title bar and docks again on the window's left edge. Floating groups stack into sets: dropped on the upper or lower half of another group (a line shows where), a group joins it right above or below it, and the set's groups stack top to bottom, each showing one panel, moving together by the top group's title bar, whose × docks them all; a group dragged by its tab strip leaves the set (`window.panel.float {below, above}`). A double-click on a tab collapses a floating group to its tabs (`collapsed`). Sets dropped on the left edge of the icon column or of another dock column dock as dock columns (a multi-column dock, `window.panel.dock {column}`), whose groups stack, tab and tear out as a floating set's do, and whose top strip floats the column again. Floating panels, their sets and the Tools panel's position are kept with the preferences and in user workspaces, and clamped into a smaller window. Not done: panels in separate OS windows, docking a panel into the default tabbed group it doesn't belong to, reordering tabs by dragging, collapsing a set or dock column to icons, dock columns right of the default dock, resizing floating panels and the groups of a set.
   - The dock's double arrow collapses Properties, Layers and Libraries to icons at the top of the icon column, popping out like the other icon panels, and expands them again (`window.collapseDock`, kept with the preferences and in saved workspaces). The icon column's own arrow (expanding the icon panels into a full column) is not done.
   - Czech render-site labels: desktop/web file filters, diagnostics, suggested preset names and contributor tooltips/plurals now use catalog lookups; gradients have Czech adjective agreement and printer marks have a separate context (PDF/Print and Summary). Search, Home, toolbar/panel tooltips and Preferences already look up their labels. Every complete catalog has rows for these labels and for the Roman and Japanese printer mark styles. Evidence: `i18n::tests::czech_render_site_follow_up`, `i18n::tests::complete_catalogs_translate_render_site_labels`, `dialogs::tests_modal::printer_mark_labels_are_distinct_from_language_names`; see [localization-parity.md](localization-parity.md).