Docs + a11y: document Cashu wallet seed storage/backup as tracked accepted-risk in SECURITY.md (F1 follow-up); add skip-to-content link and main landmark id (UX U4)

Co-authored-by: shakespeare.diy <assistant@shakespeare.diy>

SECURITY.md

@@ -247,13 +247,51 @@ oversights:
   no real protection (the decryption key would live in the same JS context). The
   meaningful mitigation is preventing script injection (CSP shipped in the bundle
   so it also covers mobile + no-`innerHTML` discipline) and offering
-  extension/bunker login for users who want keys off the page.
+  extension/bunker login for users who want keys off the page. (The Cashu wallet
+  *seed* is a partial exception — see "Known limitations — Cashu wallet" below.)
 - **Broad `connect-src`/`img-src`.** Necessary for user-chosen relays, mints,
   and media hosts. This permits data exfiltration *if* script execution is ever
   achieved — which is why preventing XSS is the top priority.
 - **`style-src 'unsafe-inline'`.** Required by the CSS framework; CSS-injection
   defense is handled by input sanitization rather than CSP.
 
+### Known limitations — Cashu wallet
+
+The built-in Cashu ecash wallet is **native-only** (browsers block most mints via
+CORS). Its BIP-39 mnemonic is the master key to all ecash funds, so it gets
+stronger handling than other secrets:
+
+- **Seed storage location (per platform).**
+  - **Native (Android / iOS):** the mnemonic is stored in the OS-backed secure
+    store — Android **KeyStore** / iOS **Keychain** — via
+    `capacitor-secure-storage-plugin` (`src/lib/secureStorage.ts`), **not** in
+    plaintext `localStorage`. Existing users on an older build are migrated
+    transparently on first read (the legacy plaintext `cashu-mnemonic` copy is
+    moved into secure storage and the plaintext removed).
+  - **Web:** falls back to `localStorage` (same accepted-risk posture as `nsec`
+    above). The wallet is effectively native-only in practice, so web plaintext
+    exposure is a minor surface.
+  - Ecash **proofs** (the spendable tokens) still live in IndexedDB
+    (`cashu-wallet`) in plaintext on device — a stolen unlocked device can spend
+    them, exactly like cash. Loss of the seed without a backup = permanent loss.
+
+- **Backup behavior (Android / native).** A freshly auto-generated seed sets a
+  `needsBackup` flag that forces a **mandatory, non-dismissable backup step**
+  (`RecoveryPhraseDialog` in `requireConfirm` mode): the user must reveal the 12
+  words and explicitly confirm ("I've written it down") before the wallet is
+  usable. This prevents a silently generated seed from being left un-backed-up —
+  clearing app storage without a backup means the funds are unrecoverable. The
+  phrase can be re-viewed any time from the wallet ("View Recovery Phrase").
+
+- **Planned / status.** Secure-storage migration is **shipped** (native seed now
+  in Keychain/KeyStore). Remaining accepted risk: proofs-at-rest in IndexedDB and
+  the web-only `localStorage` fallback. Encrypting proofs on-device is tracked as
+  a future hardening item but offers limited value while the key must live in the
+  same JS context (same reasoning as `nsec` above).
+
+  Relevant files: `src/hooks/wallet/useCashu.ts`, `src/lib/secureStorage.ts`,
+  `src/components/wallet/RecoveryPhraseDialog.tsx`.
+
 ---
 
 ## For auditors: where to look first
@@ -266,7 +304,7 @@ oversights:
 | Rich text rendering | `src/components/NoteContent.tsx` |
 | Login / key handling | `src/components/auth/`, Nostrify login (`nostr:login`) |
 | Lightning / NWC | `src/hooks/useNWC.ts`, `src/components/WalletModal.tsx` |
-| Cashu ecash (native only): proofs in IndexedDB `cashu-wallet`, seed in `cashu-mnemonic` | `src/hooks/wallet/useCashu.ts`, `src/lib/platform.ts` |
+| Cashu ecash (native only): proofs in IndexedDB `cashu-wallet`; seed in OS Keychain/KeyStore (secure storage), localStorage fallback on web | `src/hooks/wallet/useCashu.ts`, `src/lib/secureStorage.ts`, `src/lib/platform.ts` |
 | Admin authority / moderation / takedowns | `src/lib/admins.ts`, `src/hooks/useAdminModeration.ts`, `src/hooks/useDeleteEntity.ts`, `src/components/DeleteEntityDialog.tsx`, `src/components/GearSwapItemCard.tsx` |
 | CSP & security headers | `index.html` (`<meta>` — covers mobile), `public/_headers`, `netlify.toml`, `public/boot-guard.js` |
 | Native wrapper (same bundle) | `capacitor.config.ts` (`webDir: 'dist'`) |

src/components/Layout.tsx

@@ -484,6 +484,14 @@ export function Layout({ children, noPadding }: LayoutProps) {
 
   return (
     <div className="min-h-screen bg-black text-white">
+      {/* Skip-to-content — visually hidden until focused, so keyboard and
+          screen-reader users can jump past the nav on every page. */}
+      <a
+        href="#main"
+        className="sr-only focus:not-sr-only focus:fixed focus:top-3 focus:left-3 focus:z-[100] focus:rounded-lg focus:bg-[#ccff00] focus:px-4 focus:py-2 focus:text-sm focus:font-black focus:uppercase focus:tracking-wide focus:text-black focus:shadow-lg"
+      >
+        Skip to content
+      </a>
       {/* Desktop Header */}
       <header className="hidden md:flex sticky top-0 z-50 border-b border-red-500/30 bg-black/95 backdrop-blur">
         <div className="container flex items-center justify-between h-16 px-4">
@@ -967,7 +975,7 @@ export function Layout({ children, noPadding }: LayoutProps) {
 
       {/* Main Content — extra bottom padding on mobile so the fixed bottom nav never overlaps */}
       {/* noPadding: full-bleed pages (map) manage their own height */}
-      <main className={noPadding ? undefined : "pb-[calc(5rem+env(safe-area-inset-bottom,0px))] md:pb-6"}>
+      <main id="main" className={noPadding ? undefined : "pb-[calc(5rem+env(safe-area-inset-bottom,0px))] md:pb-6"}>
         {children}
       </main>