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, whatever decimals a token declares; the balance check reads
a token balance to its first 18 places, the most an amount can have. A token
holding below 0.000001 is still left off the lists as dust, except for a token
the user tracks. 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 +45,22 @@ 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,
+  whatever decimals a token declares; the balance check reads a token balance to
+  its first 18 places, the most an amount can have. 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