feat(a11y): R9 focus foundation — glass shadow fix, focus-visible ring, highlight/selected split

S1 — recipe foundation:
- Move .glass/.glass-card/.glass-menu into @layer components so a JSX shadow-* utility overrides the
  glass box-shadow again; the ~9 dead shadow-* on glass surfaces (modals, menus, save-bar, chat) now
  paint their intended drop-shadow. Perf-mode and :hover overrides stay unlayered so they still win.
- Add a .selectable recipe: the hover tint menu/list rows already use, PLUS the app-wide
  focus-visible ring that was missing on non-input controls (the biggest a11y gap).
- Split the popover roving highlight from the selected state (AnchoredPopover): a selected/checked
  PopoverItem now gets a ring ONLY, so its resting paint (bg-accent fill / text-accent / checkbox)
  survives being highlighted instead of downgrading to a 35% tint or being recoloured. Fixes the 4
  review findings deferred from R10-S0 generically; drops the leftover per-consumer data-active ring
  in tableHeader.

S2 — worst offenders:
- Account-menu trigger + switch/add/sign-out rows adopt .selectable (visible keyboard focus).
- Saved-views rows: group-focus-within reveals the hover-only action buttons for keyboard users;
  aria-current on the active row; focus-visible ring on every button.
- The 3 invisible-focus inputs get an accent focus ring: SearchBar + ShareDialog via focus-within on
  the glass pill/row, AboutCell via focus-visible on the trigger.

Deferred to a later R9 sprint: the full unification of all ~14 selection idioms, a shared <NavItem>,
and .is-current/.chip recipes (no consumer yet in this pass).

Verified: tsc/eslint/prettier/knip/vitest (79) green; browser computed-style checks confirm the glass
shadow now wins, the focus outlines render (2px accent), and the split renders (selected ring-only,
unselected fill).

frontend/src/components/tableHeader.tsx

@@ -158,7 +158,7 @@ function FilterFunnel<T>({
               checked={!filterOn}
               onSelect={() => onApply(col.key, "")}
               label={t("datatable.all")}
-              className={`text-left text-xs px-2 py-1.5 rounded-md outline-hidden transition data-active:ring-1 data-active:ring-accent ${
+              className={`text-left text-xs px-2 py-1.5 rounded-md outline-hidden transition ${
                 !filterOn ? "bg-accent text-accent-fg" : "text-muted hover:bg-card"
               }`}
             >
@@ -171,7 +171,7 @@ function FilterFunnel<T>({
                 checked={value === o.value}
                 onSelect={() => onApply(col.key, o.value)}
                 label={o.label}
-                className={`text-left text-xs px-2 py-1.5 rounded-md outline-hidden transition data-active:ring-1 data-active:ring-accent ${
+                className={`text-left text-xs px-2 py-1.5 rounded-md outline-hidden transition ${
                   value === o.value ? "bg-accent text-accent-fg" : "text-muted hover:bg-card"
                 }`}
               >