Dashboard chrome: an icon picker, one segmented shape, a rail without bars

- IconPicker replaces the three icon selects (rail icon, icon-widget rule,
  "Otherwise"): the glyphs in a grid, and a button that clears back to none —
  which a Radix SelectItem could never offer.
- ModePicker/StylePicker drop out in favour of a shared ui/Segmented, the same
  sliding-thumb shape RangePicker and the widget-side control already wear.
- PanelRail draws no scrollbars at all: hiding them also takes back the gutter
  a vertical bar claimed from a column exactly as wide as its buttons, which is
  what pushed a horizontal bar under them.
- The panels dialog can re-pair one screen (POST /panels/{id}/unpair) without
  deleting the panel it hangs on.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018tULRZJUkZsw7rMJ3h4xvu

frontend/src/components/Dashboard/PanelRail.tsx

@@ -60,7 +60,14 @@ export function PanelRail({
       aria-label="Dashboards on this panel"
       data-testid="panel-rail"
       className={cn(
-        "pointer-events-auto absolute inset-y-4 left-4 z-10 flex w-12 flex-col items-center gap-1 overflow-y-auto rounded-lg border border-border bg-card/80 p-1 shadow-e2 backdrop-blur-md",
+        "pointer-events-auto absolute inset-y-4 left-4 z-10 flex w-12 flex-col items-center gap-1 rounded-lg border border-border bg-card/80 p-1 shadow-e2 backdrop-blur-md",
+        // More dashboards than the column is tall still scroll, but no bar is
+        // ever drawn: a wall panel is swiped, and there is no room for one
+        // anyway. `w-12` less `p-1` either side is exactly the 40px button, so
+        // a classic vertical bar claiming its gutter is what pushed the buttons
+        // out sideways and put a horizontal bar under them — `overflow-y` alone
+        // computes `overflow-x` to `auto` rather than leaving it visible.
+        "overflow-y-auto [scrollbar-width:none] [&::-webkit-scrollbar]:hidden",
         className,
       )}
     >