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

An amount below the 4-decimal display floor now extends to its first significant digit on the approval and confirmation screens, instead of stating a real transfer, allowance or swap Min. received as 0.0000. The rule had been implemented three times; all three now share src/shared/amountDisplay.js, which holds the plain truncation and the floored variant side by side. History and balance lists keep the unfloored rule, pinned by test.

TODO.md

@@ -68,6 +68,22 @@ but the review is broader than any of them.
   unchanged — the README said "nothing under `dist/` that the build did not
   write", which was broader than that. Documentation only; no executable line
   changed.
+- 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, the 4-decimal truncation still printed a small amount as
+  `0.0000` — 1 base unit of an 18-decimal token, 500 of an 8-decimal one — so a
+  real transfer, allowance or swap was stated as nothing on the one screen whose
+  job is to say what is being authorized, and a swap's `Min. received` claimed
+  the user might receive nothing. Three copies of that truncation existed; they
+  now share `src/shared/amountDisplay.js`. Everything the approval and
+  confirmation screens render (`src/popup/views/approval.js`,
+  `src/shared/uniswap.js`) extends to the first significant digit when the
+  truncated figure would otherwise read as zero, keeping the amount in token
+  units rather than switching to base units mid-line. The history and balance
+  lists (`src/shared/transactions.js`) keep the unfloored rule, which is out of
+  scope by the issue's definition of done. `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