feat(databases): Import Data modal — samples, CSV/JSON files, and URL loads

[dawsontoth]

Closes #1292.

What

Replaces the bare-bones Import CSV modal with an Import Data flow modeled on the issue's mockups, with three methods:

Method	Backend operation
Sample Datasets — three bundled starter datasets (Dogs, Books, Animals), plus Random Data generation for the selected table	csv_data_load / insert
Import from File — CSV or JSON upload (was CSV-only)	csv_data_load / insert
Load from URL — CSV fetched by the instance	csv_url_load

UX details

• Front and center: a second entry point in the Databases sidebar (next to "Create a Table"), so import is available even on an empty database — the key onboarding moment the issue is about. The table-toolbar button ("Import CSV" → "Import Data") opens the same modal prefilled with the current table and defaults to the file method; the sidebar defaults to samples.
• Random Data: when the target table already exists and has columns beyond the primary key/system timestamps, the dataset picker offers a "Random Data" option (with a 1–1000 row-count input). Values are driven by each column's declared type, with column-name heuristics (email, name, age, price, boolean-ish, dates, …) for schemaless tables; the primary key is omitted so Harper assigns it.
• Tables are created automatically (primary key id) when the target doesn't exist, with an inline heads-up alert. Picking a sample dataset auto-fills its suggested table name.
• Job tracking: CSV loads return a job_id that the old modal ignored ("Please wait a few moments then refresh the table"). The new flow polls get_job until the job completes, then refreshes the table tree and navigates to the imported table — data is just there when the modal closes. Job errors (e.g. malformed CSV) surface as real error toasts.
• Sample datasets are bundled into the app via ?raw imports (~6 KB total), so they work on self-hosted/air-gapped instances that can't fetch external URLs.

Scope notes

The mockup's Connect to Database (mongodump, .zip, live sync) has no supporting operations in Harper today, so it's intentionally left out rather than shipped as a dead-end UI; the method picker makes it easy to add once the backend exists. import_from_s3 is also a candidate for a follow-up method.

Testing

• New unit tests: get_job polling (waitForJob), JSON upload parsing, random-record generation (types, heuristics, skipped columns), and sample-dataset CSV validity (format, unique ids, consistent columns). Full suite green; tsc, oxlint, dprint clean.
• Verified end-to-end against a stage test cluster: the Dogs sample import created data.dog and loaded 15 records (create_table → csv_data_load → get_job polling → auto-navigate), and Random Data then inserted 7 generated rows into that table with sensible per-column values. Also exercised both entry points, method switching, dataset auto-fill, and the new-table alert.

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request replaces the CSV-only import modal with a comprehensive ImportDataModal that supports importing data from local CSV/JSON files, remote URLs, and pre-loaded sample datasets. It also introduces utility functions for parsing JSON records, a job polling mechanism to track asynchronous imports, and corresponding unit tests. Feedback on these changes includes improving the resilience of the job polling loop against transient network errors, handling potential FileReader failures when uploading files, and adding an error handler to the import mutation to provide user feedback on failures.