feat(tui): responsive table layout for all tabs

Extract shared computeTableLayout() into table_helpers.go — takes column
definitions with short/full headers, min/max widths, and a flex column
that absorbs surplus space. All tabs now use it:

- Alerts: CONFIG column is flex, NAME/TYPE/SENT expand with width
- Maint: TITLE column is flex, TYPE/MONITORS/STATUS/dates expand
- Nodes: NAME column is flex, REGION/LAST SEEN/VERSION expand
- Users: PUBLIC KEY column is flex, USERNAME expands
- Sites: uses same colDef type (keeps special dual-flex for NAME+HISTORY)

Headers auto-switch short/full based on available width across all tabs.

internal/tui/tab_sites.go

@@ -341,23 +341,16 @@ type tableLayout struct {
 }
 
 func (m Model) computeLayout() tableLayout {
-	type colDef struct {
-		short    string
-		full     string
-		minWidth int
-		maxWidth int
-	}
-
 	cols := []colDef{
-		{"#", "#", 4, 4},
-		{"", "", 0, 0}, // NAME (dynamic)
-		{"TYPE", "TYPE", 8, 10},
-		{"STATUS", "STATUS", 8, 10},
-		{"LAT", "LATENCY", 7, 10},
-		{"UP%", "UPTIME", 8, 8},
-		{"", "", 0, 0}, // HISTORY (dynamic)
-		{"SSL", "SSL", 5, 5},
-		{"RT", "RETRIES", 5, 9},
+		{"#", "#", 4, 4, false},
+		{"", "", 0, 0, false}, // NAME (special)
+		{"TYPE", "TYPE", 8, 10, false},
+		{"STATUS", "STATUS", 8, 10, false},
+		{"LAT", "LATENCY", 7, 10, false},
+		{"UP%", "UPTIME", 8, 8, false},
+		{"", "", 0, 0, false}, // HISTORY (special)
+		{"SSL", "SSL", 5, 5, false},
+		{"RT", "RETRIES", 5, 9, false},
 	}
 
 	numCols := len(cols)