Clarify language support and model availability by task

[brittain9]

Problem

The language selector and Manage Models rail make it hard to tell whether a language has dictation, translation, or read-aloud coverage. In a live translation smoke test, selecting Filipino appeared to offer no models even though Natural translation has broad language coverage.

Explore

• Show task-specific availability when a language is selected, including installed versus downloadable models.
• Decide whether a language with no dictation model should remain visible globally, be annotated, or be scoped out of a dictation-only picker.
• Ensure translation source and target controls accurately distinguish supported pairs from model availability.

Non-goals

• Do not hide languages simply to reduce the list without a capability rule.
• Do not add language search. The language rail should be independently scrollable; that ergonomic fix is part of PR Add production Tencent HY-MT translation #412.

Acceptance criteria

A user can choose a language and immediately understand which local tasks it supports, what model is needed, and whether that model is already installed.