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/gridColumns.test.ts

@@ -14,6 +14,21 @@ describe("columnsFor", () => {
     expect(columnsFor(260, 700, null)).toBe(2);
   });
 
+  // The GUTTERS have to be in the count, not just in itemWidthFor. Every other assertion in this file
+  // survives 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 two loops compare columnsFor
+  // against itself, so they cancel any shared change (E10 review round 1). These are the exact-fit
+  // boundaries where the two formulas first disagree: an n-column row needs n*minCol + (n-1)*GAP, so
+  // at exactly that width the naive form loses the last column and wastes a third of the row.
+  it("counts the gutters, not one gap per column", () => {
+    const exactly3 = 3 * 260 + 2 * GRID_GAP; // 828
+    expect(columnsFor(260, exactly3, null)).toBe(3);
+    expect(columnsFor(260, exactly3 - 1, null)).toBe(2);
+    const exactly2 = 2 * 260 + GRID_GAP; // 536 — the FIRST width where the naive form is wrong
+    expect(columnsFor(260, exactly2, null)).toBe(2);
+    expect(columnsFor(260, exactly2 - 1, null)).toBe(1);
+  });
+
   it("never returns 0 columns, however narrow", () => {
     expect(columnsFor(260, 10, null)).toBe(1);
     expect(columnsFor(260, 0, null)).toBe(1);