fix(e10): review round 1, part 2 — the docs the epic invalidated, and three contracts made real

Finishes round 1. Part 1 took the runtime bugs; this takes the prose that
had gone false and the three "the comment says so" gaps behind it.

CLAUDE.md is auto-loaded into every agent session and ui/README.md is
not, so an architecture paragraph that still described a flat components
bag was the most expensive stale sentence in the repo: the next agent
adding a popover reads it, sees no platform layer, and writes the generic
component into components/ — the invisible-layer failure this epic exists
to end. It now names components/ui/ and says the boundary is lint-enforced.
The canonical-patterns table gained the three rows the epic established:
ui/Button over a hand-rolled <button>, a keymap row over a private keydown
listener, and disabled-when-busy vs inert-when-inapplicable.

ui/README.md's inventory was written in this branch's first commit and
never learned about CommandPalette, ContextMenu/useContextMenu,
KeyboardHelp or gridColumns — in a section whose own closing rule is that
a second component painting the same thing is how the last inconsistency
happened. It also claimed "everything is rem-derived" while VirtualGrid's
minCol/GRID_GAP are deliberately px; that is now stated as the exception
it is, and a new section records which promises here are tests rather
than sentences.

Three contracts stopped being prose:

- A text-less Button must carry a name. "Required for iconOnly" was a doc
  comment, which is why the migration shipped two nameless icon buttons
  through eight green lanes. It is a union now, covering both spellings
  (iconOnly, and size="icon"/"icon-sm"), satisfied by label, aria-label or
  title. It caught one more on the way in: Banner's dismiss took an
  optional dismissTitle, so it could render unnamed — it falls back to
  common.close now.
- gridColumns' gutter arithmetic. Every existing assertion survived
  replacing floor((w + GAP) / (minCol + GAP)) with the naive
  floor(w / (minCol + GAP)): the fixed widths agree, the capped cases are
  cap-bound, and the loops compare columnsFor against itself. Pinned at
  the exact-fit boundaries where the two first disagree (536 and 828).
- ShareDialog's link row. Only Copy was migrated in S1, leaving one 2rem
  button between two 1.75rem ones — and on a coarse pointer .touch-target
  grew only the migrated one to 2.75rem while the destructive Revoke
  stayed small.

Also: the maxCols docstring said 8 columns of 384px where the test it
shipped with asserts 369.

frontend/src/components/ui/Banner.tsx

@@ -1,4 +1,5 @@
 import type { ReactNode } from "react";
+import { useTranslation } from "react-i18next";
 import type { LucideIcon } from "lucide-react";
 import { X } from "lucide-react";
 import { Button } from "./Button";
@@ -34,12 +35,16 @@ export default function Banner({
   children: ReactNode;
   action?: { label: string; onClick: () => void };
   onDismiss?: () => void;
+  /** Accessible name (and tooltip) for the dismiss button. Optional at the call site; the component
+   *  falls back to the generic "Close" rather than shipping an unnamed icon button — a nameless
+   *  control is not an acceptable default, and `Button`'s type now says so. */
   dismissTitle?: string;
   /** `"status"` for a bar that APPEARS in response to something the user just did, so a screen
    * reader announces it. Left unset for the permanent bars (demo, version): those are part of the
    * page when it loads, and announcing them on every visit is noise, not information. */
   role?: "status";
 }) {
+  const { t } = useTranslation();
   const c = TONES[tone];
   return (
     <div
@@ -62,7 +67,7 @@ export default function Banner({
           size="icon-sm"
           className="ml-auto"
           onClick={onDismiss}
-          title={dismissTitle}
+          title={dismissTitle ?? t("common.close")}
           icon={<X className="w-4 h-4" />}
         />
       )}