Fix WebDAV auto sync conflict handling

[xwil1]

Summary

This PR improves WebDAV auto-sync conflict handling to avoid automatically overwriting remote changes.

Changes

• Track snapshot IDs and localized error keys in sync status.
• Add three-way sync state detection for WebDAV auto sync:
  • local-only changes can auto upload
  • remote-only changes pause auto sync and require manual resolution
  • local and remote changes pause auto sync and require manual resolution
  • matching local and remote snapshots refresh sync status without re-uploading
• Use WebDAV ETag preconditions for automatic uploads to avoid overwriting remote updates during upload.
• Add a WebDAV conflict dialog that lets users choose between:
  • using the local version to overwrite the cloud
  • using the cloud version to overwrite local data
• Show clearer auto-sync paused guidance in settings.
• Add snapshot/error-key tracking and remote manifest checks for S3 auto sync.
• Add translations for zh/en/zh-TW/ja.
• Add frontend test coverage for the conflict dialog and auto-sync guidance.

Scope

• Affects WebDAV/S3 cloud sync only.
• Manual upload and manual download keep their existing explicit overwrite behavior.
• Does not affect provider configuration writes, local routing, transformer routing, usage statistics, or session management.

Tests

• pnpm format:check
• pnpm typecheck
• pnpm test:unit
• pnpm build:renderer
• cargo fmt --manifest-path src-tauri/Cargo.toml --check
• git diff --check

[farion1231]

@codex review

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve legacy manifest baselines for auto-sync

For profiles that synced before this change, the stored status has last_remote_manifest_hash but no last_*_snapshot_id. When those users make a local-only edit after upgrading and the remote manifest is unchanged, last_synced_snapshot_id(status) is None, so this path classifies the safe upload as Conflict and pauses WebDAV auto-sync indefinitely until the user resolves it manually. The S3 guard still falls back to last_remote_manifest_hash, but this WebDAV state assessment ignores that existing baseline; pass/compare the remote manifest hash or migrate the status so upgraded users do not get false conflicts.

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