Overlay: two lines stay English in a Czech browser #75

Found while preparing #74 (the overlay's words after LazurioPlatform#460).

The overlay speaks Czech where the person's browser does (prefersCzech), with formal address and "Prostředí" for the place. Two of its lines are English for everyone:

• apps/web/src/lazurio/LazurioEnvironmentBrowser.tsx, the empty panel when the Environment offers no browser: "Browser unavailable" / "This environment does not offer its browser now. Its tabs come back when it does." (also a lowercase "environment").
• apps/web/src/lazurio/LazurioPromptDraft.tsx, the toast when a prepared prompt cannot open: "Could not open the prepared prompt" / "Nothing was added to the composer."

Proposed words, Czech first:

• "Prohlížeč není k dispozici" / "Toto Prostředí teď prohlížeč nenabízí. Jeho záložky se vrátí, až ho zase nabídne." and "Browser unavailable" / "This Environment does not offer its browser now. Its tabs come back when it does."
• "Připravené zadání se nepodařilo otevřít" / "Do pole zprávy se nic nepřidalo." and the English unchanged.

Both are overlay files (rows 9 and 6 of the runbook's Overlay table), so no upstream file or allowlist changes. Not part of #74, which only removes developer information and keeps one concern.