Add a "Translate" toolbar button to the dictionary search sheet

[megabulk]

What this does

Adds a "Translate" button to the dictionary search toolbar (next to Furigana) that sends the current context sentence directly to iOS's built-in Translate sheet, in one tap — no need to switch to Edit mode, manually select all the text, and choose Translate from the contextual menu.

This follows up on the request in #61.

Why this approach

Rather than introducing a new machine-translation engine, this reuses Apple's own system Translate sheet — the same one already reachable today via Edit mode → select text → Translate from the contextual menu. It's just skipping the manual-selection step. No new translation dependency, no new UI surface to maintain.

Behavior notes

• If the user is mid-edit in the context box, it translates the in-progress edited text rather than the original OCR'd text.
• iPad/iPhone only for now. I looked into Mac (both Mac Catalyst and "Designed for iPad" mode) and ran into real platform limitations — .translationPresentation is unavailable at compile time under Mac Catalyst, and unreliable under "Designed for iPad." Rather than ship something half-working, the button is hidden on Mac entirely. Happy to revisit Mac support in a follow-up if there's interest.

Testing

Tested on iPad Simulator and a physical iPad, with real dictionary content (Jitendex + SudachiDict tokenizer) imported, across both the normal display and in-progress-edit cases.