security: decrypt and sign dApp approvals in the popup (closes #157)

The dApp transaction and signature approval paths sent the user's plaintext
password to the background over runtime.sendMessage and decrypted there. Both
now decrypt in the popup, where the password is typed, and put only the signed
artifact on the wire: the raw signed transaction, or the signature. Neither the
password, the recovery phrase, the xprv nor the private key crosses the
messaging boundary any more. This matches what the popup-side eth_sendTransaction
path in confirmTx.js already did.

The popup runs the same sequence ethers' own sendTransaction() runs internally
(populateTransaction, then signTransaction), so nonce, gas, fee and chain id
population are unchanged. The background keeps broadcast and approval
resolution, and when the popup cannot produce an artifact it reports the error
over the same message so the requesting page still gets a failure rather than
hanging.

Moving the secret out of the background must not turn the background into a
blind relay, so it re-derives the signer from the artifact and checks it
against the approval it is holding before acting: shared/approvalVerify.js
asserts that a raw transaction is the approved transaction signed by the
approved address, and that a signature covers the approved payload and recovers
to the approved address.

A wrong password is now caught in the popup before anything is sent, so it
fails with an inline full-sentence error and leaves the pending approval alive
to retry; previously it reached the background and destroyed the approval.
Rejection still resolves with EIP-1193 code 4001, and approvals still survive
popup close and reopen.

Removes the four standing TODO(security) markers, now that the flaw is gone.

TODO.md

@@ -23,6 +23,10 @@ get review, merge to main.
 
 # Completed Steps
 
+- 2026-08-09: dApp approval signing moved into the popup — the password no
+  longer crosses the extension messaging boundary; the background broadcasts and
+  resolves approvals only, and verifies the signed artifact against the approval
+  it holds (#157).
 - 2026-07-07 Adopted scripts-to-rule-them-all: `script/` entrypoints, Makefile
   shims, README Entrypoints section
 - 2026-03-01: About well in settings with build info and easter egg (in flight