fix: never render a nonzero approval amount as zero (closes #322)

formatTxValue() truncates to 4 decimal places, per README.md's Display
Consistency rule. With the token's true scale resolved, an amount below 0.0001
still printed as 0.0000: 1 base unit of an 18-decimal token, 500 base units of
an 8-decimal one. On the dApp approval screen — and on the wait/success/error
screens, which carry that same string forward as txInfo.amount — a real
transfer or allowance was therefore stated as nothing.

The copy of formatTxValue() in src/popup/views/approval.js now extends to the
first significant digit when the first 4 decimals would all be zero and the
value is not: 0.000000000000000001 DAI, not 0.0000 DAI. Extra precision was
chosen over falling back to base units so the number stays in token units, the
same unit as the symbol printed beside it; the base-unit rendering already on
this screen means "the scale is unknown", and reusing it for a known scale
would blur the two. A genuine zero still renders 0.0000, and an amount at or
above the floor is untouched.

The 4-decimal rule stays. The separate copy of formatTxValue() in
src/shared/transactions.js, which serves the history list, is deliberately not
changed: the history detail view already shows exact precision, and balance
lists and history are out of scope here.

tests/approvalDisplayFloor.test.js drives decodeCalldata() and asserts both the
displayed line and the rawValue the confirmation screens carry, at 6, 8 and 18
decimals and across every scale from 0 to 30, plus four cases pinning the
unchanged truncation. Against this tree with the source change reverted, 4 of
its cases fail — 1 base unit at 18 decimals gives "0.0000 DAI" where
"0.000000000000000001 DAI" is expected, 500 base units at 8 decimals gives
"0.0000" where "0.000005" is expected.

make check green: 42 suites, 844 tests; verify-build 39 cases; check-censored
152 files; eslint and prettier clean in the pinned container.

TODO.md

@@ -44,6 +44,20 @@ but the review is broader than any of them.
 
 # Completed Steps
 
+- 2026-08-23: An amount below the 4-decimal display floor no longer reads as
+  zero on the approval screens
+  ([#322](https://git.eeqj.de/sneak/AutistMask/issues/322)). With the token's
+  true scale resolved, `formatTxValue()` still truncated a small amount to
+  `0.0000` — 1 base unit of an 18-decimal token, 500 of an 8-decimal one — so a
+  real transfer or allowance was stated as nothing on the one screen whose job
+  is to say what is being authorized. Its copy in `src/popup/views/approval.js`
+  now extends to the first significant digit when the first 4 decimals would all
+  be zero and the value is not, keeping the amount in token units rather than
+  switching to base units mid-line. The 4-decimal rule is otherwise unchanged,
+  the copy in `src/shared/transactions.js` that serves the history list is
+  untouched, and `README.md`'s Display Consistency section records the
+  exception.
+
 - 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