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/lib/useFocusKeeper.ts

@@ -29,6 +29,21 @@ function firstFocusable(node: Node | null, fromEnd = false): HTMLElement | null
   return null;
 }
 
+/** Move the focus off `el`, which is about to go, to its nearest focusable neighbour — the next one,
+ *  else the previous one, widening to the parent's neighbours — so its removal does not drop the
+ *  focus to <body>. For a removal the caller makes itself (a chip's ×) anywhere, including a modal
+ *  or a side panel, where the keeper below does not watch. False when nothing was found. */
+export function focusNeighbour(el: HTMLElement): boolean {
+  for (let at: HTMLElement | null = el; at && at !== document.body; at = at.parentElement) {
+    const hit = firstFocusable(at.nextSibling) ?? firstFocusable(at.previousSibling, true);
+    if (hit) {
+      hit.focus({ preventScroll: true });
+      return true;
+    }
+  }
+  return false;
+}
+
 // The element the keeper last put the focus on, while it still holds it. A Back that swaps the page
 // or a sub-view removes the held control too, and the keeper's stand-in then sat where
 // `restoreNavPosition` looks for an untouched <body>: the restore gave up and Back left the focus on