feat: bulk dictionary import via paste or file upload

[NathanSkene]

Summary

• Adds "Import" button to the Custom Dictionary view (both empty and populated states)
• Opens a dialog with a textarea for pasting words and a file upload button (.txt, .csv, .json)
• Live preview shows new word count and duplicate count before confirming
• Parses input by splitting on newlines, commas, and tabs to handle all common list formats
• Adds i18n translation keys for all new UI strings (English; other locales use fallback defaults)

Closes #460

Test plan

• Open Settings → Custom Dictionary (empty state) → verify "Import" link appears below example tags
• Click Import → paste multi-line words → verify preview shows correct counts → confirm adds words
• Click Import → upload a .txt file with one word per line → verify words are parsed and added
• Click Import → upload a .json file with a string array → verify words are parsed and added
• Verify duplicate words are skipped (add some words first, then import a list containing some of them)
• Verify words persist after closing and reopening the app (localStorage + SQLite sync)
• Verify Import button also appears in the populated state (next to "Clear all")
• Verify Cancel button closes dialog without adding words

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