Add on-device per-message translation

[rexbron]

Summary

User-initiated, on-device translation for received messages using Apple's Translation framework — no server round-trip, no third-party API.

• Right-click or long-press a message → Translate runs the message body through TranslationSession and swaps the bubble to the translated text.
• Right-click → Show Original reverts.
• Translated bubbles get a translate-glyph badge; hovering surfaces the original text.
• The system download prompt for missing language pairs is reached via SwiftUI's .translationTask (the only public entry point that triggers it).
• Up to 3 translations run in parallel through a small slot pool — clicking Translate on several rows back-to-back no longer queues serially.
• Source language is auto-detected with NLLanguageRecognizer; messages already in the user's current locale are skipped silently (no badge, no menu item).

Implementation notes

• New RelayKit/Translation/MessageTranslator.swift handles source detection and the readable-language heuristic. It deliberately does not drive translation directly — the framework only prompts for downloads through .translationTask.
• TimelineViewModel exposes a FIFO queue (pendingTranslationQueueVersion + claimNextTranslation() + runTranslation(for:translate:)). Atomicity comes from @MainActor isolation.
• TimelineView renders 3 invisible TranslationSlot subviews; each owns its own Configuration @State and .translationTask, so slots are independent.
• TimelineRowView's Equatable was updated to include row.translation and translationsVersion, otherwise the NSTableView cell-recycling path would skip body re-evaluation after a translation completed.
• Translation is presentation-only — copy / reply / quote / edit all continue to operate on the original body. Not persisted across launches.

Test plan

• Right-click a non-locale message → Translate appears; clicking it badges the row and swaps the bubble text.
• Hovering a translated row shows the original body in the tooltip.
• Right-click again → Show Original reverts.
• Triggering Translate on three messages in quick succession runs all three in parallel (no serial wait).
• First-time translation of an uninstalled language pair surfaces the system download prompt.
• Same-locale messages have no Translate item and no badge.
• Image / membership / redacted rows have no Translate item.
• Network off: previously-installed language pair still translates (proves on-device path).
• Edit lands after translation: stale translation clears.

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