feat: add hex editor popover for BLOB columns

[datlechin]

Summary

• Add hex editor popover that opens on double-click of BLOB/binary column cells in the data grid
• Popover shows read-only hex dump (address | hex bytes | ASCII) and editable hex input with live validation
• Follows the same pattern as the existing JSON editor popover (PopoverPresenter, commitPopoverEdit)

Changes

• New: HexEditorContentView.swift — SwiftUI popover with HexDumpDisplayView (read-only NSTextView) and HexInputTextView (editable NSTextView with coordinator)
• Modified: DataGridView+Click.swift — added BLOB type check in handleDoubleClick() before multiline fallthrough
• Modified: DataGridView+Popovers.swift — added showBlobEditorPopover() method
• New: HexEditorTests.swift — 31 tests covering formatting, parsing, validation, round-trips, and edge cases

Test plan

• All 31 unit tests pass (HexEditorTests)
• Connect to MariaDB/MySQL with BLOB columns → double-click BLOB cell → hex editor appears
• Edit hex → Save → cell updates with new value
• Double-click NULL BLOB cell → empty editor → type hex → Save
• Cancel discards changes
• Invalid hex shows "Invalid hex" in red, Save button disabled
• Large BLOB (>10KB) shows truncated hex dump

Summary by CodeRabbit

• New Features

  • Hex editor popover for BLOB/binary cells now opens on double-click; BLOBs are excluded from inline editing.
  • Popover shows a read-only hex dump plus editable hex input with live validation, byte-count, truncation/read-only states, and Cancel/Save (keyboard shortcuts). Edits commit only when content changes and multiple hex input formats are accepted.

• Documentation

  • Added Hex Editor docs and clarified grid BLOB display/size rules.

• Tests

  • Added comprehensive hex formatting/parsing and truncation tests.