feat(datagrid): omit hidden columns from the table browse query

[datlechin]

Feature

Requested: choose per table which columns to load before fetch, so a table with one heavy column opens fast.

Built on the existing per-table column visibility (you asked "why not based on column visibility?" — agreed, it's cleaner). Hiding a column now also leaves it out of the browse query, so it isn't fetched. This is a clean narrower SELECT col_a, col_c, <pk> — no truncation, no aliasing, none of the corruption risk that removed the auto-SUBSTRING in #1373.

Behavior

• Hidden = not fetched. A hidden column is omitted from the query. Hide a heavy column → the table opens/loads faster. Reuses the existing column-visibility popover + per-table persistence.
• Primary key is always fetched even if hidden, so row identity / editing / UPDATE keep working.
• Toggling re-runs the query (debounced) so the change takes effect; showing a column re-fetches it.
• Scoped at open: when a table has persisted hidden columns, the first query is already scoped (column list comes from the cached, async SQLSchemaProvider.getColumns), so re-opening is fast — no SELECT * round-trip first.
• The visibility popover lists the full schema (not the fetched result), so an omitted column can still be toggled back on.

Trade-off (called out): hide/show is no longer a purely cosmetic instant toggle — it re-runs the query.

Implementation

• ColumnFetchScope.selectColumns(schema:hidden:primaryKeys:) — pure, tested: full schema minus hidden, PK always kept, nil when nothing is omitted.
• TableQueryBuilder selectColumns: projection (nil ⇒ SELECT *).
• MainContentCoordinator+ColumnFetchScope — per-table schema-column cache (from getColumns), selectColumns(for:), requeryWithColumnScope, and the full-schema list for the picker. Cache cleared on schema reload / reset.
• Wired through FilterCoordinator (rebuild/apply/clear), the open path, and the visibility toggles.

Tests

ColumnFetchScopeTests covers the projection: drop hidden, keep order, PK-always-retained, empty schema, hide-everything-no-PK, and unknown columns. swiftlint --strict clean.