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/index.html

@@ -119,6 +119,15 @@
                         Import private key
                     </button>
                 </div>
+                <div class="mt-1 text-xs text-muted">
+                    Have an extended private key (xprv)?
+                    <button
+                        id="btn-add-wallet-import-xprv"
+                        class="underline cursor-pointer bg-transparent border-none text-fg text-xs font-mono p-0"
+                    >
+                        Import xprv
+                    </button>
+                </div>
             </div>
 
             <!-- ============ IMPORT PRIVATE KEY ============ -->
@@ -170,6 +179,55 @@
                 </button>
             </div>
 
+            <!-- ============ IMPORT XPRV ============ -->
+            <div id="view-import-xprv" class="view hidden">
+                <button
+                    id="btn-import-xprv-back"
+                    class="border border-border px-2 py-1 hover:bg-fg hover:text-bg cursor-pointer mb-2"
+                >
+                    &lt; Back
+                </button>
+                <h2 class="font-bold mb-2">Import Extended Private Key</h2>
+                <p class="mb-2">
+                    Paste your extended private key (xprv) below. This will
+                    import the HD wallet and scan for used addresses.
+                </p>
+                <div class="mb-2">
+                    <input
+                        type="password"
+                        id="import-xprv-key"
+                        class="border border-border p-1 w-full font-mono text-sm bg-bg text-fg"
+                        placeholder="xprv..."
+                    />
+                </div>
+                <div class="mb-2" id="import-xprv-password-section">
+                    <label class="block mb-1">Choose a password</label>
+                    <p class="text-xs text-muted mb-1">
+                        This password encrypts your key on this device. You will
+                        need it to send funds.
+                    </p>
+                    <input
+                        type="password"
+                        id="import-xprv-password"
+                        class="border border-border p-1 w-full font-mono text-sm bg-bg text-fg"
+                    />
+                </div>
+                <div class="mb-2" id="import-xprv-password-confirm-section">
+                    <label class="block mb-1">Confirm password</label>
+                    <input
+                        type="password"
+                        id="import-xprv-password-confirm"
+                        class="border border-border p-1 w-full font-mono text-sm bg-bg text-fg"
+                    />
+                </div>
+                <button
+                    id="btn-import-xprv-confirm"
+                    class="border border-border px-2 py-1 hover:bg-fg hover:text-bg cursor-pointer"
+                >
+                    Import
+                </button>
+            </div>
+
             <!-- ============ MAIN VIEW: ALL WALLETS & ADDRESSES ============ -->
             <div id="view-main" class="view hidden">
                 <!-- active address headline -->