Add Claude data-export importer (parity with ChatGPT import)

[thoscut]

Adds an importer for Claude data exports (the ZIP downloaded from claude.ai → Settings → Privacy → "Export data"), mirroring the existing ChatGPT export import end to end.

• New src/claude-export.ts: preflight + resumable import job + dedup ledger for Claude's format — single conversations.json array, flat linear chat_messages (human/assistant, ISO timestamps), rich content blocks (text/thinking/tool_use/tool_result/voice_note, token_budget skipped), and text-only attachments via extracted_content. Also imports design_chats/*.json through the same pipeline via a normalized thread model; projects/ metadata is counted but not turned into sessions.
• New src/export-archive.ts: shared zip/folder reader, media sniffing and small helpers factored out of chatgpt-export.ts so both importers reuse the same battle-tested archive reader instead of duplicating it.
• Register the new import-only 'claude-export' source across types, config, session titles and transfer-import bookkeeping.
• Server API: /api/claude-export/import/{preflight,status,start}; source filter + display label.
• Electron file picker (main + preload) and full web UI: import card, JS handlers, source color/badge/filter, and en/zh-CN/ja strings.
• scripts/claude-import-smoke.js (+ npm run smoke:claude-import) covering parsing, block mapping, attachments, and re-run dedup.

Verified end to end against a real 319-conversation export (312 non-empty threads imported, correct roles/blocks/timestamps, re-run dedups all) and via the smoke test.

Claude-Session: https://claude.ai/code/session_01CNrWLKk3T8wC2KbwDFJtnB