KOD-19: Remove column resize UI to fix table layout bugs

[jasadams]

Summary

• Removes column resize UI (drag handles, resize drag logic, mousedown handler, CSS) to fix three bugs from Jason's human review rejection:
  1. Column widths had no markdown round-trip persistence — widths lost on serialize/parse
  2. Layout corruption after row insertion when table had fixed-layout colgroup from prior resize
  3. Phantom "Header" columns and overlapping handles caused by resize/colgroup interaction
• Keeps DocState model infrastructure (set_column_widths_at, apply_col_widths, width maintenance in insert/delete, colgroup rendering) for when resize is re-added with proper persistence
• Filed KOD-23 as follow-up ticket for column resize with markdown round-trip persistence

Closes KOD-19

Test plan

• cargo check --workspace passes
• cargo test --workspace passes (897 tests, 0 failures)
• Verify in browser: table insert handles (row/column) work correctly
• Verify in browser: column/row selection with inline delete works
• Verify in browser: right-click context menu still works
• Verify in browser: no resize handles visible (removed)
• Verify in browser: inserting rows/columns no longer causes layout corruption