feat: view private key for addresses (closes #19)

- Add 'Show Private Key' button to address detail view
- Create dedicated password verification modal with warning text
- Derive private key from HD wallet mnemonic or use directly for imported keys
- Display key in read-only well with copy button
- Add getPrivateKeyForAddress() to shared/wallet.js
- Never log, cache, or auto-clear the private key
- Document clipboard non-clearing policy in README

README.md

@@ -835,6 +835,25 @@ Currently supported:
 - [ ] Multi-currency fiat display (EUR, GBP, etc.)
 - [ ] Security audit of key management
 
+## Private Key Export
+
+The address detail view includes a "Show Private Key" button. After entering the
+wallet password, the raw hex private key is displayed and can be copied to the
+clipboard.
+
+We intentionally **do not clear the clipboard** after copying a private key:
+
+1. **User expectations**: Clipboard clearing violates the principle of least
+   surprise. Users expect their clipboard to contain what they last copied until
+   they copy something else.
+2. **Data safety**: The user may copy something else important moments later. An
+   auto-clear timer could destroy that unrelated clipboard content, causing data
+   loss far worse than the theoretical risk it was meant to mitigate.
+
+If a user chooses to display their private key, they have already been warned
+that it controls all funds at the address. Managing sensitive data on their own
+clipboard is their responsibility.
+
 ## Policies
 
 - We don't mention "the other wallet" by name in code or documentation. We're