feat: support importing multiple files at once

[komen205]

Summary

• Replaces the single-file picker in the import dialog with a multi-file picker (multiple: true)
• Files are imported sequentially to avoid database conflicts
• All results are aggregated into a single combined summary dialog
• Removes the useLocalStorage dependency for the file path (no longer needed)

Motivation

When importing a Postman data export (which produces a folder of individual collection JSON files), users had to go through the import dialog once per file. This change allows selecting all files at once.

Test plan

• Open Import dialog — button should read "Select File(s)"
• Select a single file — imports and shows summary as before
• Select multiple files — all are imported, summary shows combined totals (e.g. "3 Workspaces, 45 HTTP Requests")
• Cancel file picker — nothing happens

🤖 Generated with Claude Code

[komen205]

Tested by importing multiple Postman collections at once — worked as expected. All collections were imported and the combined summary showed the correct totals.