gui: offer to convert a Berkeley DB wallet to SQLite on first run

A first-run dialog for an existing wallet.dat with no backend choice yet:
"Convert to SQLite" exports the wallet and records the sqlite marker so
the next start opens it; "Keep Berkeley DB" records that choice so the
offer does not return; "Decide later" leaves no marker and asks again.
wallet.dat is never touched, and the change takes effect on restart --
matching the restart-to-apply model.

The conversion itself is DoConvertWalletToSQLite(), a headless-testable
function the button calls: it reuses the export path (extracted as
ExportActiveWalletToSQLite, shared with -walletsqliteexport, which keeps
its own specific messages), refuses if already on SQLite or if a
wallet.sqlite exists, and writes the marker only after a verified export.

New wallet-convert self-test proves the action end to end: build a
Berkeley DB wallet, convert, and confirm wallet.sqlite + the marker are
written, wallet.dat is left untouched, a second convert refuses, and a
flagless restart opens the converted SQLite wallet with its phrase intact.
All wallet suites pass. The modal itself is thin UI over this tested
action.

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