fix: a send the confirmation screen refuses moves nothing on it (closes #583)

Sign & Send wrote its two refusals, a network changed since review and a fee over the bound, into the errors box above the password field. That box reserves one empty line, so a refusal grew it and pushed the password field and Sign & Send down.

Each refusal now has its own box with its sentence fixed in index.html, holding its space from the first paint and only made visible, as #558 did for the balance messages. The fee refusal no longer quotes the fee; its full reason is logged. A Chrome test raises the stub node's fee only at signing and checks the password field and Sign & Send stay where they were.

Model: opus-5-5

src/popup/index.html

@@ -697,6 +697,25 @@
                     id="confirm-fee-unknown-error"
                     class="mb-2 border border-border border-dashed p-2 text-xs invisible"
                 ></div>
+                <!-- Shown when Sign & Send refuses a send for its network or
+                its fee. Each holds its space from the first paint, so the
+                refusal moves nothing. -->
+                <div
+                    id="confirm-network-changed-error"
+                    class="mb-2 border border-border border-dashed p-2 text-xs invisible"
+                >
+                    The active network changed after this transaction was
+                    reviewed, so it was not sent. Please go back and review it
+                    again.
+                </div>
+                <div
+                    id="confirm-fee-bound-error"
+                    class="mb-2 border border-border border-dashed p-2 text-xs invisible"
+                >
+                    This transaction would allow a network fee of more than this
+                    wallet will sign for, so it was not sent. Please go back and
+                    try again.
+                </div>
                 <div class="mb-2">
                     <label class="block mb-1 text-xs">Password</label>
                     <input