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.

CLAUDE.md

@@ -59,9 +59,14 @@ how it gets built. Concretely:
 `downloads/`, `youtube/` (shared by routes + scheduler + worker) → flat core (`auth`, `quota`,
 `sysconfig`, `state`, `audit`, `notifications`, `userscope`, `realtime`, `ratelimit`, `progress`,
 `security`) → `models/db/config`. **Dependency direction is one-way: core never imports routes.**
-Frontend: `main.tsx` provider tree → `App.tsx` shell → `moduleRegistry` route-chunks → components; data
-via react-query (`qk` factory), per-account `localStorage`, and server prefs; API through `lib/api/*`
-slices over `core.req<T>`.
+Frontend: `main.tsx` provider tree → `App.tsx` shell → `moduleRegistry` route-chunks → module
+components → the **platform layer `components/ui/`** (domain-free primitives: the popover engine and
+its menus, `Modal`/`Overlay`, `Button`/`form`, `DataTable`/`VirtualGrid`, `QueryState`, …). That
+boundary is one-way and **lint-enforced**, not just described: a platform component may not import a
+feature component or a domain API slice (`no-restricted-imports` in `eslint.config.js`, with its own
+test). A new generic component goes in `ui/`; see [`frontend/src/components/ui/README.md`](frontend/src/components/ui/README.md).
+Data via react-query (`qk` factory), per-account `localStorage`, and server prefs; API through
+`lib/api/*` slices over `core.req<T>`.
 
 **Not wanted** (reviewed and rejected as over-engineering for this size): microservices, event bus,
 CQRS, a repository/DI framework, a router library, a generic CRUD-router factory or base serializer.
@@ -82,6 +87,9 @@ CQRS, a repository/DI framework, a router library, a generic CRUD-router factory
 | Job status word | the `downloads/status.py` StrEnum | loose `"canceled"`/`"cancelled"` literals |
 | Current time | inline `datetime.now(timezone.utc)` | new `now_utc`/`_now` helpers |
 | FE per-account state | `usePref` | raw `readAccount`/`writeAccount` or bespoke hooks |
+| FE action button | `ui/Button` — pick a `SIZES` rung | a hand-rolled `<button className="px-3 py-2 …">` |
+| FE keyboard shortcut | a row in `lib/keymap.ts` + `dispatch()` | a private `keydown` listener with its own key literals |
+| FE menu row that is off | `disabled` only when BUSY; `inert` when it does not apply | `disabled` + a `title` nobody can reach |
 | FE control | the chip/segmented primitives (`Select` only for >6 options) | a native `<select>` |
 | FE loading/empty/error | `QueryState` / `StateMessage` | ad-hoc conditionals |