feat(grid): client-side sorting for primitive result columns

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Problem

Result grids could not reliably sort by a column:

• DataGrid (query results) rendered sort UI, but a header click emitted sortChange and QueryResultPanel re-ran the user's SQL through execute_sorted_query, wrapping it as SELECT * FROM (…) ORDER BY …. That only works for plain parsable SELECTs, re-runs potentially expensive queries on every click, and fails outright on anything sqlparser cannot parse.
• DataTableView (table data browser) had no sort support at all.

Solution

Sorting now happens in the frontend, over the rows already returned, and is limited to primitive columns (text / number / boolean / date / NULL). JSON documents, arrays and BLOBs stay unsortable — they have no meaningful client-side total ordering.

header click ─▶ useDataGridSort (SortState) ─▶ sortRowsByState(rows)
                                                  │ primitive-only comparator
                                                  ▼
                                            sortedRows (computed) ─▶ render

Sortability — a column qualifies only if its declared type is primitive (not JSON/ARRAY/BYTEA/BLOB/BINARY/…) and every returned cell is primitive. One object/array cell disqualifies the whole column. Unsortable headers render as plain labels with a tooltip, and the header context menu disables its sort entries and explains why.

Comparison — number / boolean / date / string, preferring the declared column type (so bigint returned as a string still compares numerically). Dates compare by parsed timestamp. Strings use localeCompare(…, { numeric: true, sensitivity: 'base' }). NULLs sort last ascending; descending is a pure reversal, matching PostgreSQL.

Multi-column sort — plain click cycles ASC → DESC → cleared; shift-click appends (ASC → DESC → removed). Equal rows keep their original order and the input array is never mutated.

Changes

Area	Change
src/utils/gridSort.ts	new — sortability detection, comparator, sortRowsByState, formatSortState
useDataGridSort	adds setSort / pruneSort; removes now-unused buildOrderByClause and hasActiveSort
DataGrid.vue	sortedRows is the single source of truth for rendering, row actions, copy/export and the status bar; selection is cleared when rows/columns/sort change (selection is positional)
QueryResultPanel.vue	sorting no longer re-executes; only filters still round-trip, since they change the result set itself
DataTableView.vue	clickable sortable headers with indicators; export and batch delete read sortedRows; sort resets on table/connection change
ColumnHeaderContextMenu.vue	sortable prop disables sort entries for non-primitive columns
i18n	components.dataGrid.sort.unsortable in enUS / zhCN
docs/GRID_SORT_DESIGN.md	design write-up

Known limitations

• Table browser sort is page-scoped. get_table_data paginates server-side (100/500/1000 rows per page), so only the fetched page is reordered. Cross-page sorting would need ORDER BY pushed into that command — intentionally out of scope here.
• The Rust execute_sorted_query command keeps its sort field (the frontend now always sends an empty list), so no backend change is required and the API stays compatible.

Testing

• src/__tests__/gridSort.test.ts — sortability detection, per-kind comparison, NULL placement, multi-column priority, stability, immutability.
• src/__tests__/useDataGridSort.test.ts — click/shift cycling, setSort, pruneSort.
• Full suite: 34 suites / 609 tests pass; vue-tsc --noEmit and eslint ./ clean.