DataTable: cell-cursor table with frozen columns

[meszmate]

Summary

Adds `DataTable`, a third table component for the "spreadsheet view" use case: many columns, navigable cell-by-cell, leftmost columns pinned so they stay visible while the rest of the grid scrolls horizontally.

How it differs from existing components

Component	Cursor	Columns	Best for
`Table`	row	compile-time count	small static tables
`SortableTable`	row	compile-time count	sortable lists
`DataTable` (new)	cell	runtime count	databases, log fields, CSVs

Features

• Cell-level cursor with arrow / hjkl navigation
• Page up/down, goto first/last row/column
• Frozen columns: first N columns never scroll out of view
• Horizontal scrolling for non-frozen columns when they exceed available width
• Configurable per-column width and alignment (left/center/right)
• Distinct styles for cursor cell vs cursor row vs header
• Visible "│" separator between frozen and scrolling regions

Test plan

• `zig build test` — added 6 tests covering right-clamp, down-cursor, frozen offset behavior, view content, frozen separator rendering, and column-count validation