feat(a11y): R9 focus foundation — app-wide focus-visible fallback + nav rail focus

- Global :focus-visible fallback (@layer base): one accent outline for keyboard users on every
  button/link/[role=button]/[role=tab]/summary that doesn't define its own. Non-input controls had no
  visible keyboard focus app-wide; this closes the void with a single rule. More specific focus styles
  (.selectable, an input's focus:ring, a menu item's data-active highlight) override it.
- PopoverItem now carries outline-hidden so menu items keep their data-active highlight without also
  drawing the global outline (no double ring).
- Nav rail rows adopt .selectable (via rowBase); rail chrome buttons (logo/pin/about/sign-out) get a
  focus-visible ring; the settings row already uses rowBase. Deduped the account trigger's selectable.

Deferred to R10: the chip/pill selected-PAINT unification (a .chip/.pill/.is-current repaint of the
~14 selection idioms) — the R10 token redefinition repaints those surfaces anyway, at less risk than a
hand migration now. R9's job (the focus/a11y foundation) is complete.

Verified: tsc/eslint/prettier/knip/vitest (79) green; browser computed-style confirms the global ring
renders (2px accent, offset 2px) on a plain button, .selectable overrides it with an inset ring, and
an outline-hidden menu button draws no global outline.

frontend/src/index.css

@@ -47,6 +47,21 @@
   }
 }
 
+/* R9 focus foundation: one accent focus ring for keyboard users on every interactive element that
+   doesn't define its own. In @layer base so any component/utility focus style (.selectable, an input's
+   focus:ring, a menu item's data-active highlight + outline-hidden) overrides it. Non-input controls
+   app-wide had no visible keyboard focus before R9; this is the single biggest a11y win. */
+@layer base {
+  button:focus-visible,
+  a:focus-visible,
+  [role="button"]:focus-visible,
+  [role="tab"]:focus-visible,
+  summary:focus-visible {
+    outline: 2px solid color-mix(in srgb, var(--accent) 60%, transparent);
+    outline-offset: 2px;
+  }
+}
+
 :root {
   --font-scale: 1.06;