enhancement: validate start ≤ end in date range forms before submitting

[ali5ter]

Summary

The splash date range form and the in-TUI date range dialog (internal/tui/splash.go) validate each date field individually with validateDate (format only), but do not check that start ≤ end. If a user enters From: 2026-05-18 and To: 2026-01-01, the form submits successfully, api.DateRange returns an error, and the TUI shows a generic error screen with no recovery path (other than quitting and relaunching).

Why it matters

The error message returned by api.DateRange ("start date 2026-05-18 is after end date 2026-01-01") is informative but unreachable in the TUI — it becomes the msg.err from dataMsg, which renders as a static error with "Press q to quit." The user has to restart the app entirely.

Suggested approach

Add a cross-field validation either:

1. In validateDate on the end field by accessing the form's start value (huh supports cross-field validation via closures).
2. Or as a post-submit check before firing the splashDoneMsg / data fetch command — if start > end, redisplay the date form with a pre-populated error message.

For the in-TUI dialog, catching it here is especially valuable since the user would otherwise be dumped to the unrecoverable error screen.

🤖 Generated with Claude Code on behalf of Alister