feat: CSV batch import via context menu and file drop

[careck]

Summary

Import CSV files to batch-create child notes under a parent. Each CSV row becomes a child note with fields populated from column values. Uses a hybrid mapping approach: auto-mapping from schema field definitions by default, with optional register_csv_import Rhai hook for custom mapping logic.

This is the inverse of the text export views feature (#170) and extends the register_drop hook system (#200).

Design Spec

Full spec in the CSV Import section of docs/superpowers/specs/2026-05-13-register-drop-design.md

Entry Points

• Context menu: Right-click a note → "Import CSV..." → file picker → preview → import
• File drop / paste: Drop a .csv file on a tree node → routes to CSV import flow (not generic file drop)

Key Features

• Hybrid mapping (Approach C): If the child schema has a register_csv_import handler, use it. Otherwise, auto-map CSV column headers to schema field names with type coercion.
• Preview dialog: Shows row count, sample rows, mapped/ignored columns, warnings (coercion failures), and errors (missing required fields). Import is blocked when errors exist.
• Type coercion: text as-is, number/rating parsed to f64, boolean from true/false/1/0, date from ISO 8601 + common formats, select validated against options list.
• Schema selection: auto-selected if parent allows only one child schema, otherwise picker dialog.

Script API

register_csv_import("Book", "Book List", |parent, row| {
    let child = create_child(parent.id, "Book");
    set_title(child.id, row["title"]);
    set_field(child.id, "author", row["author"] ?? "Unknown");
    commit();
});

Scope

• Core engine: register_csv_import hook registration and dispatch, auto-mapping engine with type coercion
• Desktop crate: csv crate for parsing, two new Tauri commands (preview_csv_import, execute_csv_import)
• Frontend: CsvImportPreviewDialog.tsx, context menu entry, CSV file drop routing, i18n keys

Dependencies

• Extends feat: register_drop hook for external data (URLs, images, text) #200 (register_drop hook system)
• Inverse of Text export views: CSV, JSON, Markdown via Rhai scripts #170 (text export views)