fix: make saveState() a read-modify-write merge instead of a full-blob overwrite (closes #304)

Every extension page (the toolbar popup, a dApp approval window, the
background's backgroundRefresh()) holds its own in-memory `state`, loaded
once, and showView() saves on every navigation. saveState() wrote the
entire state blob, so any second page that saved overwrote whatever
another page had written since -- a whole wallet, name, addresses and
encrypted secret included, with no attacker and no unusual input.

saveState() now re-reads storage, diffs the persisted fields against a
deep-cloned baseline snapshot taken at this page's last
loadState()/saveState(), and writes only the fields that differ. Every
other field is carried forward from storage in its loaded-and-normalized
shape (normalizePersisted(), shared with loadState()), so a legacy or
malformed record a load has always self-healed in memory keeps getting
written back even on a save that touched something unrelated.
showView() fires saveState() without awaiting it, so two saves from the
SAME page can be in flight at once; a FIFO queue serializes them.

Deliberately not done, a documented deviation from the plan on the
issue: the live `state` of a field this page does not own is not
rehydrated from what another page wrote, only the persisted record is.
Adopting a concurrently-written value into `state` reintroduced the same
clobber one page later, under the fire-and-forget saveState() calling
convention every view uses -- caught red by tests/txStatus.test.js.
Two writers of the same field still resolve last-writer-wins, documented
at the merge point.

tests/stateMerge.test.js covers both required cases against the real
state.js and showView(): a save from a page loaded before a wallet was
added elsewhere, and the approval-window reproduction from the issue.
Both were confirmed failing against the prior full-blob write before
this fix landed.

TODO.md

@@ -44,6 +44,32 @@ but the review is broader than any of them.
 
 # Completed Steps
 
+- 2026-08-20: A second extension page can no longer silently delete a wallet
+  ([#304](https://git.eeqj.de/sneak/AutistMask/issues/304)). `saveState()` wrote
+  the entire state blob, and every extension page — the toolbar popup, a dApp
+  approval window, `backgroundRefresh()` — holds its own in-memory `state`,
+  loaded once, with `showView()` saving on every navigation; a second page that
+  saved after a first had written something new overwrote it, no attacker or
+  unusual input required. `saveState()` is now a read-modify-write: it re-reads
+  storage, diffs the persisted fields against a deep-cloned `baseline` snapshot
+  taken at the last `loadState()`/`saveState()` on that page, and writes only
+  the fields that actually changed — everything else is carried forward from
+  storage in its loaded-and-normalized shape (`normalizePersisted()`, shared
+  with `loadState()`), so a legacy or malformed record a load has always
+  self-healed in memory keeps getting written back even on a save that touched
+  something else entirely. `showView()` fires `saveState()` on every navigation
+  without awaiting it, so two saves from the same page can be in flight at once;
+  a FIFO queue serializes them rather than letting a slow one finish after a
+  later one and re-derive a stale answer. Deliberately not done: the live
+  `state` of a field this page does not own is not rehydrated from what another
+  page wrote, only the persisted record is — adopting a concurrently-written
+  value into `state` reintroduced the same clobber one page later, caught by
+  `tests/txStatus.test.js` red. Two writers of the same field still resolve
+  last-writer-wins, documented at the merge point. `tests/stateMerge.test.js`
+  covers the two-page save and the approval-window reproduction from the issue —
+  add a wallet in one page, force a save from a second page loaded before it,
+  both wallets survive — each demonstrated failing against the unfixed full-blob
+  write.
 - 2026-08-20: A forgotten password no longer wedges the wallet
   ([#312](https://git.eeqj.de/sneak/AutistMask/issues/312)). Deleting a wallet
   was password-gated and importing its recovery phrase again was refused as a