fix: show balances and fees below 0.000001 as nonzero on the send screens (closes #343)

The stored ETH and token balances and the send-confirm screen's fee were each
cut to six decimal places, so a value below 0.000001 read as zero. Balances are
now stored exactly; a token declaring more than 18 decimals is cut to 18, the
most the balance check reads. A token holding below 0.000001 is still left off
the lists as dust, except for a token the user tracks, whose holding now
reaches the Send screen and is what the send is checked against. The send and
send-confirm screens' balances, reserve and insufficient-balance messages go
through truncateAmountNeverZero(). The send-confirm and approval screens both
render the fee through formatFee(), which prices the exact fee in USD. The
balance lists still round with toFixed(4).

Model: opus-5-5

TODO.md

@@ -45,6 +45,21 @@ but the review is broader than any of them.
 
 # Completed Steps
 
+- 2026-10-04: The Send and confirmation screens no longer show an ETH balance, a
+  token balance or a network fee below 0.000001 as zero
+  ([#343](https://git.eeqj.de/sneak/AutistMask/issues/343)). The stored balances
+  (`src/shared/balances.js`) and the confirmation screen's fee were each cut to
+  six decimal places by a rule of their own. Balances are now stored exactly,
+  except that a token declaring more than 18 decimals is stored to 18, the most
+  the balance check reads. A token holding below 0.000001 is still not listed,
+  but only for a token the user does not track, so a tracked token's holding
+  reaches the Send screen and the send is checked against it. The Send screen's
+  `Current balance`, and the confirmation screen's balance, fee, reserve and
+  insufficient-balance messages, go through `truncateAmountNeverZero()` in
+  `src/shared/amountDisplay.js`, the helper the approval screen already used.
+  The confirmation and approval screens both render the fee through
+  `formatFee()` in `src/popup/views/helpers.js`, which prices the exact fee in
+  USD, so the same fee reads the same on both, USD value included.
 - 2026-10-04: Settings lists the sites connected without "Remember", and
   removing a site there disconnects it
   ([#406](https://git.eeqj.de/sneak/AutistMask/issues/406)). Such a connection