feat(send): add a shared contact picker to the recipient field

Phase 0c. The Send recipient field only offered Paste and a tx-history
suggestion list — no way to pick a saved contact. Add a compact contacts
icon button next to Paste that opens a picker popup over the App-owned
address book; selecting a contact fills the recipient (re-validated next
frame).

- New header-only src/ui/windows/contact_picker.h: ContactPickerPopup(id,
  book, outBuf, outSz) — a reusable popup listing contacts ([Z]/[T] tag +
  label + short address) that copies the chosen address into the caller's
  buffer. Kept out of the material layer so the design-system headers stay
  free of data/ deps; reserved for Chat "new conversation" later.
- send_tab: shrink the recipient input to make room for the icon button
  (ICON_MD_CONTACTS), wire the popup to app->addressBook(). Coexists with the
  existing Paste + suggestion affordances.
- i18n: send_contacts_button tooltip.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/util/i18n.cpp

@@ -751,6 +751,7 @@ void I18n::loadBuiltinEnglish()
         // Send Tab
         strings_["pay_from"] = "Pay From";
         strings_["send_to"] = "Send To";
+        strings_["send_contacts_button"] = "Pick from contacts";
         strings_["amount"] = "Amount";
         strings_["memo"] = "Memo (optional, encrypted)";
         strings_["miner_fee"] = "Miner Fee";