refactor: unify add-wallet, import-key, and import-xprv into single view

Merge all three wallet import methods (recovery phrase, private key,
extended key/xprv) into one tabbed add-wallet view with a mode selector.
This fixes the blank import-xprv render (it was missing from the VIEWS
array) and the broken back-button navigation from the separate import
views.

- Add tab selector: Recovery Phrase | Private Key | Extended Key (xprv)
- Share password fields across all modes
- Remove separate import-key and import-xprv views and modules
- Add duplicate wallet detection for private key imports
- All tabs follow affordance policy (visible border + hover state)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/popup/views/helpers.js

@@ -13,7 +13,6 @@ const { state, saveState } = require("../../shared/state");
 const VIEWS = [
     "welcome",
     "add-wallet",
-    "import-key",
     "main",
     "address",
     "address-token",