Make auto log-out per-wallet — only signs out the wallet it's set for

The logout timer is now an account setting (stored per account in the session and
the durable directory), not a global one, and it removes only that wallet from
the device — its session entry, directory entry, cached state (xpub + seed keys),
and its vault entry — leaving other wallets untouched.

- wipeAccount(id) surgically removes one wallet; the active one falls back to the
  next account (or the unlock screen).
- Background-away timestamp drives per-account evaluation on resume/boot; an armed
  timer fires for the soonest-due wallet so a blurred-but-visible window still
  signs out on time. Overdue saved wallets are removed from the vault when it's
  unlocked (the only point we can re-encrypt it).
- New cacheKeyFor() export lets app code target one wallet's cache keys exactly
  (verified against the wallet's seed/xpub/watch keys).

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

1 files changed, 7 insertions, 0 deletions

diff --git a/src/wallet.js b/src/wallet.js
index 44694bc..b4aff8b 100644
--- a/src/wallet.js
+++ b/src/wallet.js
@@ -47,6 +47,13 @@ export function newMnemonic(strengthBits = 128) {
   return generateMnemonic(wordlist, strengthBits);
 }
 
+// The base localStorage cache key for a wallet identity (xpub, xprv, or
+// seed-id) — mirrors Wallet#_cacheKey, so app code can target a specific
+// account's cached state (e.g. to wipe it on log-out).
+export function cacheKeyFor(id) {
+  return 'btc-wallet-cache:' + hex.encode(sha256(new TextEncoder().encode(id))).slice(0, 32);
+}
+
 // Account-level xpub for a seed/key without constructing a Wallet — used to
 // verify an entered recovery phrase matches a watch-only account before
 // upgrading it to spendable.