Add Bubble Tea TUI with interactive treemap

Cell canvas renderer with frame cache and a perceptual 256-colour
quantizer; responsive geometry (large/medium/small/too-small);
squarified treemap with nested previews, LOD grouping, category
colouring and adaptive labels; spatial arrow navigation with stable
NodeID selection; zoom and breadcrumbs; modal system (info, help,
confirm, save); list, file-type, top-N, scan-info, duplicate and
comparison views; live search/filter; mouse support; move-to-trash.

internal/scan/panic.go

@@ -0,0 +1,14 @@
+package scan
+
+import "runtime/debug"
+
+// OnPanic is called if a scanner goroutine panics. The default re-panics
+// (crashing the process); the TUI replaces it so it can restore the terminal
+// before reporting. Programmer bugs are never silently swallowed.
+var OnPanic = func(v any, stack []byte) { panic(v) }
+
+func guard() {
+	if v := recover(); v != nil {
+		OnPanic(v, debug.Stack())
+	}
+}