polish(tui): responsive column hiding — 3-tier priority-based layout

Columns drop progressively as terminal narrows:
- Compact (<90): #, NAME, STATUS, LATENCY
- Medium (90-119): + TYPE, UPTIME, HISTORY
- Wide (120+): + SSL, RETRIES

Column definitions are data-driven via siteColumns slice.
Row builder uses pickCols() helper so headers and cells can't drift.

Closes #68

internal/tui/table_helpers.go

@@ -15,7 +15,10 @@ var (
 
 type StyleOverride func(row, col int) *lipgloss.Style
 
-const wideBreakpoint = 120
+const (
+	wideBreakpoint   = 120
+	mediumBreakpoint = 90
+)
 
 func (m Model) isWide() bool {
 	return m.termWidth >= wideBreakpoint