chore(ui): drop the unreachable sell branch from swap processing

[bmc08gt]

Stacked on #619 — base it there, not main. Merge that first.

#619 deletes the Currency Sell flow outright (CurrencySellAmountScreen, CurrencySellViewModel, CurrencySellConfirmationScreen, CurrencySellConfirmationViewModel, and their tests), along with the v1 LoadedContent branch of CurrencyInfoScreen where onSell lived. That was the only swapType: .sell call site — Convert replaces it and carries its own case — so nothing constructs a sell any more.

SwapProcessingViewModel still branched on it in four places. This drops case sell from SwapType and the arms it fed in title, subtitle, navigationTitle, and trackTransaction, and updates the two tests that asserted a fourth case and the "Selling TestCoin" title.

Convert keeps reporting through Analytics.tokenSell — it still disposes of the source token — so only the unreachable UI path goes. Session.sell is untouched; Convert calls the same machinery.

Conflict note

feat/usdc-to-dollars-conversion-graphic (commit f3f580a8) touches this same title switch to suppress the "of " suffix when a convert lands in the reserve, and rewrites the targetMint doc comment. Preserve that branch's case .convert where targetMint == .usdf arm and its comment, minus the now-stale "nil on the sell path" clause.