docs: dedicated Arch Linux host+client guide

Every other distro has a full Host Setup page; Arch only had table rows. Add
docs/arch.md (signed pacman binary repo: key import + repo + install, GPU
prereqs, service/linger, web console, client, PKGBUILD appendix), slot it into
the nav after fedora-kde, and point the install/client tables at it. Update the
client-install rows from 'from the PKGBUILD' to the binary repo now that it exists.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

clients/decky/src/ui.tsx

@@ -0,0 +1,46 @@
+// Shared UI primitives for the fullscreen page + modals. The one rule that keeps every row
+// looking consistent: a Field's action(s) always sit right-aligned, with real space between
+// them and the label text — never hugging it.
+//
+// Decky lays a Field out as `[ label .......... children ]`. When the children container is
+// grown (`childrenContainerWidth="max"`, which we want so multi-button clusters have room), a
+// bare `fit-content` button LEFT-aligns inside that grown container and ends up pressed against
+// the label with the space wasted to its right. Wrapping the action(s) in `RowActions` pushes
+// them to the right edge and evenly spaces multiples — the same treatment every row now gets.
+import { Focusable } from "@decky/ui";
+import { CSSProperties, FC, ReactNode } from "react";
+
+export const RowActions: FC<{ children: ReactNode }> = ({ children }) => (
+  <Focusable
+    style={{
+      display: "flex",
+      gap: "0.5em",
+      justifyContent: "flex-end",
+      alignItems: "center",
+    }}
+  >
+    {children}
+  </Focusable>
+);
+
+// A single action button sized to its content (not the gamepad-UI default of 100% width), with
+// a floor so short labels ("Pair", "Remove") don't render as tiny nubs and every row's button
+// reads at the same weight.
+export const actionButton: CSSProperties = {
+  width: "fit-content",
+  minWidth: "7em",
+  flexShrink: 0,
+};
+
+// Square icon-only button (details ⓘ, header back arrow). Needs an explicit height or the zero
+// padding collapses it to the icon's line height.
+export const iconButton: CSSProperties = {
+  width: "40px",
+  minWidth: "40px",
+  height: "40px",
+  padding: 0,
+  flexShrink: 0,
+  display: "flex",
+  alignItems: "center",
+  justifyContent: "center",
+};