Add per-table "hide chrome/headers when unfocused" clean-look option

New Settings checkbox: while a table isn't the selected canvas object,
its title bar, column letters and row numbers disappear entirely -- not
just visually, their reserved space collapses too, so the table shrinks
to just its cell grid. Clicking back into it (e.g. any cell) reselects
it and brings everything back.

TableObject.hideChromeUnfocused (store/types.ts) + setTableHideChromeUnfocused
(document.ts) + io/json.ts normalization, following the existing
per-object appearance-setting pattern. CanvasObject.vue gates its title
bar on a new showChrome computed and passes its existing `selected` prop
down to TableGrid.vue (previously only took tableId) so it can make the
same call for its own headers.

TableGrid.vue's HEAD_W (a plain constant) becomes a `headW` computed,
and the existing `headerH` computed now returns 0 for both whenever
hidden -- sufficient on its own since rowTop()/colLeft() already sum
those two values as their base offset and are the single source every
other position calculation in the file (sticky-frozen offsets, the
in-cell editor overlay, clip/fill marquees, scroll-into-view, row
virtualization) already goes through.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/io/json.ts

@@ -341,6 +341,7 @@ function normaliseObject(o: SheetObject): SheetObject {
       locale: localeOrUndef(o.locale),
       bg: colorOrUndef(o.bg),
       fg: colorOrUndef(o.fg),
+      hideChromeUnfocused: boolOrUndef(o.hideChromeUnfocused),
     };
   }
   if (o.kind === 'text') {