harden: escape every interpolation into popup innerHTML, and add default-src to both manifests (closes #307)

A hostile ERC-20's symbol() reached an innerHTML string unescaped, and neither
manifest declared default-src, so an attacker deploying a token with 1,000+
holders and airdropping one unit could render a full-viewport cross-origin
iframe over the wallet's own UI, on screens where the user types their
password.

escapeHtml is now a pure string replace over & < > " ' — the old version
round-tripped through textContent, which escapes neither quote, while already
being used inside data-copy="...". All 19 files in src/popup/views/ were
audited: beyond the reported symbol site, the explorer-supplied directionLabel
in all three transaction lists, wallet.name, addr.ensName, the blockie data:
URIs and two ad-hoc quote-only escapes were also unescaped. Explorer URLs now
go through one helper that percent-encodes the path segment.

Both manifests add default-src 'self', frame-src 'none', form-action 'none' and
base-uri 'none'. Three loosenings are pinned in tests/manifest.test.js and
justified in README.md: style-src 'unsafe-inline' (39 static style attributes;
Firefox implements neither style-src-attr nor 'unsafe-hashes'), img-src data:
(blockies), connect-src https: http: (user-configurable RPC).

Note frame-src 'none' blocks a frame loading, not the element existing, so the
zero-iframe assertion is a claim about the escaping alone; the test asserts the
element count and the literal rendered text separately, taking the count before
any click an overlay could intercept.

Verified: make check 39 suites / 811 tests, test-e2e 55/55 including the
WebAssembly-under-CSP assertion, test-e2e-firefox 8/8, zero CSP violations
asserted rather than merely unobserved. Reverting only balanceLine's
interpolation reproduces the attack as 2 iframes on the address screen.

TODO.md

@@ -44,6 +44,28 @@ but the review is broader than any of them.
 
 # Completed Steps
 
+- 2026-08-20: A hostile ERC-20 symbol no longer renders as live HTML in the
+  popup ([#307](https://git.eeqj.de/sneak/AutistMask/issues/307)). A token
+  symbol is whatever the contract's `symbol()` returns, the block explorer
+  passes it through unfiltered, and `balanceLine()` interpolated it into an
+  `innerHTML` string — so a token with the 1,000 holders the spam filter asks
+  for, airdropped to the victim, could paint a full-viewport cross-origin iframe
+  over the wallet's own UI, on the screens where the user types their password.
+  `escapeHtml` moved to `src/shared/html.js` as a pure string replace over `&`,
+  `<`, `>`, `"` and `'`: the old implementation round-tripped through a detached
+  element's `textContent`, which does not escape quotes, and it was already
+  being used inside `data-copy="..."`. Every interpolation into an `innerHTML`
+  string across `src/popup/views/` was audited, not just the reported one — the
+  transaction lists' direction label, the wallet name and ENS name in the Home
+  list, the `href` in the explorer link, and the confirmation screen's warning
+  line were all unescaped as well. Both manifests now declare
+  `default-src 'self'` with `frame-src 'none'`; the four directives that had to
+  stay looser than `'self'` are named and justified in the Content Security
+  Policy section of README.md, and `tests/manifest.test.js` pins the whole set
+  exactly. A display cap of 12 characters bounds the symbol, matching the bound
+  `lookupTokenInfo()` already applied on the contract-read path. Not repurposed
+  for any of this: `isSpoofedSymbol()`, which answers a different question and
+  would have been the wrong control.
 - 2026-08-20: A page asking which chain the wallet is on is told the chain the
   user is actually on ([#317](https://git.eeqj.de/sneak/AutistMask/issues/317)).
   `eth_chainId` and `net_version` answered from `currentNetwork()`, which reads