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/NavSidebar.tsx

@@ -322,7 +322,7 @@ export default function NavSidebar({
               ref={acctPop.refs.setReference}
               {...acctPop.getReferenceProps()}
               title={slim ? `${name} · ${roleLabel}` : undefined}
-              className={`${rowBase} ${slim ? "justify-center px-0" : ""} text-muted hover:text-fg hover:bg-card`}
+              className={`${rowBase} selectable ${slim ? "justify-center px-0" : ""} text-muted hover:text-fg hover:bg-card`}
             >
               <span className="relative shrink-0">
                 <AvatarImg
@@ -367,7 +367,7 @@ export default function NavSidebar({
                       key={a.id}
                       onClick={() => switchTo(a.id)}
                       title={t("header.account.switchTo", { name: a.display_name ?? a.email })}
-                      className="w-full flex items-center gap-2.5 px-2 py-1.5 rounded-lg hover:bg-card transition"
+                      className="selectable w-full flex items-center gap-2.5 px-2 py-1.5 rounded-lg hover:bg-card transition"
                     >
                       <AvatarImg
                         src={a.avatar_url}
@@ -393,14 +393,14 @@ export default function NavSidebar({
                     clearActiveAccount();
                     window.location.href = "/auth/login";
                   }}
-                  className="w-full flex items-center gap-2 text-sm px-2 py-2 rounded-lg text-muted hover:text-fg hover:bg-card transition"
+                  className="selectable w-full flex items-center gap-2 text-sm px-2 py-2 rounded-lg text-muted hover:text-fg hover:bg-card transition"
                 >
                   <UserPlus className="w-4 h-4" />
                   {t("header.account.addAccount")}
                 </button>
                 <button
                   onClick={logout}
-                  className="w-full flex items-center gap-2 text-sm px-2 py-2 rounded-lg text-muted hover:text-fg hover:bg-card transition"
+                  className="selectable w-full flex items-center gap-2 text-sm px-2 py-2 rounded-lg text-muted hover:text-fg hover:bg-card transition"
                 >
                   <LogOut className="w-4 h-4" />
                   {t("header.account.signOut")}