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, and a token holding cut to zero was dropped, so a
value below 0.000001 read as zero. Balances are now stored exactly, whatever
decimals a token declares, and every nonzero token holding is kept; the balance
check reads a token balance to its first 18 places. The balance lists, the
send-screen token selector, the address total and the remove-address warning
leave out a holding below 0.000001 themselves, through isBelowOneMillionth().
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.

Model: opus-5-5

TODO.md

@@ -45,6 +45,24 @@ 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, and a token holding cut to zero was
+  dropped. Balances are now stored exactly, whatever decimals a token declares,
+  and every nonzero token holding is kept; the balance check reads a token
+  balance to its first 18 places, the most an amount can have. The balance
+  lists, the send-screen token selector, the address total and the
+  remove-address warning leave out a holding below 0.000001 themselves, as
+  before. 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: The flash line keeps to the one line it reserves at any message
   length ([#252](https://git.eeqj.de/sneak/AutistMask/issues/252)). A message
   that wrapped pushed the whole screen below it down. `#flash-msg` no longer