fix(a11y): chips, grips, the panel tab and Back to top keep the focus somewhere real

- F5: a chip's × pressed from the keyboard hands the focus to the next chip (else the previous one,
  else what follows) before it unmounts — in DismissibleChip, so all five chip rows get it
  (`focusNeighbour` in lib/useFocusKeeper).
- F4: a drag grip names what it lifts (`common.order.dragNamed`: panel groups, personal views);
  the two generic "Drag to reorder" keys are gone.
- F6: the side panel's edge tab is out of the Tab order while the card covers it, names its panel,
  and a keyboard pin moves the focus into the card.
- F7: Back to top is no Tab stop while hidden, and a keyboard press leaves the focus on the page.

frontend/src/components/ui/SidePanel.drawer.dom.test.tsx

@@ -78,12 +78,31 @@ describe("SidePanel · a mouse on a wide window", () => {
   it("pins from the tab, as before", () => {
     stubMedia({ coarse: false, narrow: false, wide: true });
     const { onToggleCollapse } = mount();
-    fireEvent.click(screen.getByRole("button", { name: "sidebar.pin" }));
+    fireEvent.click(screen.getByRole("button", { name: "sidebar.pinNamed:Filters" }));
     expect(onToggleCollapse).toHaveBeenCalledOnce();
     expect(screen.queryByRole("dialog")).toBeNull();
   });
 });
 
+// E28 review F6: the pinned (or peeking) card covers the tab, which only goes transparent — yet it
+// stayed a Tab stop, an invisible control the focus landed on, named just "Unpin panel".
+describe("SidePanel · the tab under an open card", () => {
+  it("is out of the Tab order while the card is open, and names its panel", () => {
+    stubMedia({ coarse: false, narrow: false, wide: true });
+    globalThis.ResizeObserver ??= class {
+      observe() {}
+      unobserve() {}
+      disconnect() {}
+    } as unknown as typeof ResizeObserver;
+    mount(false); // pinned
+    const tab = screen.getByRole("button", { name: "sidebar.unpinNamed:Filters", hidden: true });
+    expect(tab.tabIndex).toBe(-1);
+    cleanup();
+    mount(true);
+    expect(screen.getByRole("button", { name: "sidebar.pinNamed:Filters" }).tabIndex).toBe(0);
+  });
+});
+
 describe("SidePanel · on touch", () => {
   it("opens a drawer from the tab instead of pinning, and Esc closes it", () => {
     stubMedia({ coarse: true, narrow: false, wide: false });