feat: add JSON Table preview for response pane

[khemthit]

Summary

Adds a Table view for JSON responses in the response pane as a tabular
alternative to the existing tree view. Tree remains the default; users opt in
with a Tree/Table toggle inside the JSON preview area. The choice persists
per tab.

Features

• Array-of-objects renders as a single table; columns are the union of
  top-level keys, preserved in first-appearance order.
• Single objects render as a 2-column key | value table.
• Nested objects and arrays render inline as recursive sub-tables — every
  level of the structure is visible at once, no clicking required.
• Click a column header to sort the rows (asc → desc → none). Sort picks
  a numeric, boolean, or localeCompare string comparator per column.
  null values sort last in both directions.
• Debounced text filter (200 ms) matches case-insensitive substrings
  across every column, including nested-cell previews.
• Pagination at 100 rows per page; hidden when there is only one page.
• Right-click any cell to copy its value — primitives copy as-is, nested
  values copy as pretty-printed JSON.
• Filter / sort / page reset automatically when the response data changes.

Implementation notes

• The detection logic (`flatten.ts`) and the filter/sort/paginate logic
  (`tableState.ts`) are pure modules with 31 vitest unit tests in the
  existing `node` test environment.
• React components (`TableView`, `ValueRenderer`, `ModeToggle`,
  `FilterInput`, `JsonTablePreview`) are thin shells over those pure helpers.
• `ModeToggle` is self-styled so it renders identically in both contexts
  (Tree mode overlay and Table mode toolbar).
• Theme tokens used throughout (`theme.background.`, `theme.border.`,
  `theme.colors.text.*`) match the convention in sibling files.
• A new `responseJsonPreviewMode: 'tree' | 'table' | null` field on the tab
  schema persists the user's choice per request tab. Default `null` resolves
  to `tree`, so existing tabs are unaffected.
• No new runtime dependencies and no new dev dependencies.