feat: add xprv wallet import support

Add the ability to import an existing HD wallet using an extended
private key (xprv) instead of a mnemonic phrase.

- New 'xprv' wallet type with full HD derivation and address scanning
- New importXprv view with password encryption
- Updated getSignerForAddress to handle xprv wallet type
- Added xprv link to the add-wallet view
- Allow adding derived addresses for xprv wallets

Closes #20

src/popup/views/home.js

@@ -239,7 +239,7 @@ function render(ctx) {
         html += `<div>`;
         html += `<div class="flex justify-between items-center bg-section py-1 px-2" style="margin:0 -0.5rem">`;
         html += `<span class="font-bold cursor-pointer wallet-name underline decoration-dashed" data-wallet="${wi}">${wallet.name}</span>`;
-        if (wallet.type === "hd") {
+        if (wallet.type === "hd" || wallet.type === "xprv") {
             html += `<button class="btn-add-address border border-border px-1 hover:bg-fg hover:text-bg cursor-pointer text-xs" data-wallet="${wi}" title="Add another address to this wallet">+</button>`;
         }
         html += `</div>`;