feat(settings): add toggle to protect conversation history from auto-deletion

[daymade]

Summary

• Add a "Keep conversation history" toggle in Settings → General, below "Skip Claude Code first-run confirmation"
• When enabled, sets cleanupPeriodDays: 99999 in ~/.claude/settings.json to prevent auto-deletion
• Toggle syncs from Claude's actual settings.json on load, reflecting manual edits

Root Cause

Claude Code silently deletes conversation transcripts after 30 days by default. Users are unaware until they lose data. Multiple upstream issues:

• anthropics/claude-code#4172
• anthropics/claude-code#22547
• anthropics/claude-code#23710

Changes

Rust backend (3 files):

• claude_mcp.rs: set_cleanup_period_days() / clear_cleanup_period_days() / get_cleanup_period_days() — reads/writes ~/.claude/settings.json
• commands/plugin.rs: 3 Tauri commands
• lib.rs: Command registration

Frontend (9 files):

• WindowSettings.tsx: ToggleRow with History icon (amber-500)
• useSettings.ts: Auto-save logic following skipClaudeOnboarding pattern
• useSettingsForm.ts: Field init + on-load sync from actual Claude settings
• settings.ts / schemas/settings.ts / types.ts: Type definitions
• en.json / zh.json / ja.json: i18n (3 locales)

Why This Is Safe

• Follows the exact same pattern as skipClaudeOnboarding (proven, shipped)
• Target file (~/.claude/settings.json) is the official Claude Code settings file
• Incremental JSON merge — preserves all existing fields
• Setting cleanupPeriodDays: 99999 is the documented workaround
• Toggle OFF removes the field entirely, reverting to Claude's default behavior

Testing

• cargo check passes cleanly
• 13 files changed, 186 insertions, 1 deletion

Closes #1623

🤖 Generated with Claude Code

[farion1231]

@codex

[chatgpt-codex-connector]

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Reviewed commit: 3174941fdd

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[daymade]

Hi @farion1231, I've pushed a fix commit (7101e187) addressing the two Codex review comments plus one additional race condition found during cross-model review (Claude + Codex adversarial challenge).

Changes in the new commit

useSettings.ts — auto-save handler:

• Removed stale data?. comparison for both skipClaudeOnboarding and keepConversationHistory. The updates !== undefined guard is sufficient, and the old comparison could skip API calls when data hadn't refetched after rapid toggles.

useSettings.ts — explicit save handler:

• Kept data?. comparison (added clarifying comments). This prevents a premature clearTranscriptProtection() call when the async init hasn't completed yet — the form default is false, so saving before init would silently wipe an existing protection setting.

useSettingsForm.ts:

• Added userTouchedTranscriptRef — once the user manually toggles keepConversationHistory, the async getTranscriptProtection() read no longer overwrites the form state. This prevents confusing UI flips from late-resolving promises.

All changes pass pnpm typecheck. Ready for re-review.