fix: the ETH line reads unknown at an address no refresh has read (closes #736)

An address's ETH balance is now null until a refresh has read it. That covers
a new address, a new wallet, addresses a wallet scan finds, and every address
after a network switch.

Its line reads `quantity unknown`, Home's balance `unknown (ETH)`, and Send
`Current balance: unknown (ETH)`. Max says the balance is unknown. Send is no
longer refused as zero. The address has no total, and DeleteAddress keeps its
warning.

A refresh also deletes `balanceNotFetched` only when it read every address in
the stored record.

Model: opus-5-5

README.md

@@ -1025,6 +1025,7 @@ removing the address only stops the wallet tracking it. It explains the way
 back: delete the whole wallet in Settings and import its backup again. The scan
 after an import finds only addresses with on-chain activity. When the address
 holds anything, the screen lists the holdings and the USD total under a warning.
+An ETH balance that no refresh has read counts as holding something.
 
 "Remove Address" removes the address and its site permissions. It returns to
 Home with the flash message "Address removed." The wallet's `nextIndex` is not
@@ -1212,13 +1213,25 @@ there in place of a zero. It does so after each of these:
 - an address or a wallet is added;
 - the network is switched.
 
-This lasts until a refresh has read the token's balance at every address.
+This lasts until a refresh in the popup has read the token's balance at every
+address.
+
+An address's ETH balance is `null` until a refresh has read that address. This
+holds for a new address, and for every address after a network switch. While it
+is `null`:
+
+- the balance lists show "quantity unknown";
+- Home's ETH balance reads `unknown (ETH)`;
+- Send reads `Current balance: unknown (ETH)`, and "Max" fills in nothing;
+- "Send" is not refused for a zero balance;
+- the address has no total;
+- DeleteAddress shows its warning.
 
 Only a whole number from 0 to 80 counts as a token's decimals. A token's
-`decimals` is stored only as a source reported it, never guessed. A `null`
-balance means the decimals are unknown, never that the balance is zero. While
-"Use Blockscout" is off, the bundled list and the tracked tokens may lack a
-token's symbol or decimals. The approval and status screens then add a line
+`decimals` is stored only as a source reported it, never guessed. A token's
+`null` balance means the decimals are unknown, never that the balance is zero.
+While "Use Blockscout" is off, the bundled list and the tracked tokens may lack
+a token's symbol or decimals. The approval and status screens then add a line
 under that token saying that Blockscout's report is not used.
 
 The wallet's own ERC-20 send is encoded with the decimals the screen showed. At
@@ -1254,6 +1267,7 @@ unpriced holding was left out. `formatAddressTotal()` renders that result the
 same way on every screen:
 
 - on Sepolia, or before the first price fetch: no total;
+- while the address's ETH balance is unknown: no total;
 - everything priced: `Total: $5,500.00`;
 - part priced: `Total: $3,000.00 plus unpriced tokens`;
 - nothing priced but something held: `Total: unpriced tokens only`.