feat(frontend): reorder selected Log Explorer columns - #2921

Log Explorer allows users to select and remove columns, but does not provide a way to reorder them.

This change adds:

• Drag handles for reordering selected columns.
• Alt + Left/Right keyboard shortcuts.
• Fixed positions for the expand, severity and Time columns.
• Preservation of column widths across reordering.
• Tests covering drag events, keyboard movement, removal and invalid drops.

Reordering updates the existing column state used by URL state and CSV export. Saved-search persistence is provided separately by #2911.

Validation:

• 105 frontend tests passed.
• Production build passed.
• Combined with fix(frontend): respect configured timezone in logs and alerts #2910 and fix(log-explorer): preserve saved search columns #2911: 117 tests and production build passed after resolving the shared useMemo dependency hunk.
• Native browser drag interaction still requires manual verification.

The existing lockfile required a temporary local repair for missing @emnapi entries to run validation. That unrelated change is not included.