Implement neighbor-sum-avoidance polish + new computing-pi interactive

neighbor-sum-avoidance (T2-T6):
- Start button now reads just "Start" (no mode suffix).
- Available numbers and shuffled preview arrangements render pre-start
  so the arena isn't empty while the user is still deciding.
- Mode switch + count slider stay mounted once active; they're just
  disabled, so starting no longer causes a layout jump.
- Guided-mode instructions expanded to call out light grey edges, red
  violation arcs, and the green valid-neighbor line explicitly.
- Draggable numbers honor isActive for the draggable attribute and
  onDragStart, showing opacity-60 + cursor-not-allowed when locked.

computing-pi (new interactive under Miscellany):
- N parallel first-passage simulations: toss until heads > tails,
  record the fraction. Average × 4 converges to π.
- Header row: runs slider | live average × 4 | Start/Reset.
- Tile grid uses a mask-gradient marquee so long sequences scroll
  their tail while staying readable.
- Click any tile once all runs complete to see the full toss sequence
  in a Dialog.
- Safety cap of 1000 tosses per run (first-passage time has infinite
  expectation, so without the cap a pathological run could block).
- Registered in interactives.ts + InteractiveRenderer + the parallel
  interactive-components map. Filled in three-bank-accounts in the
  latter while there, since it was missing.

admin-data.json updated to mark T2-T6 done with remarks, remove the
implemented Computing Pi idea, and add an implementation note.

src/lib/interactive-components.ts

@@ -43,4 +43,6 @@ export const componentMap: Record<string, LazyImport> = {
   "rent-division-puzzle": () => import("@/components/interactives/RentDivisionPuzzle"),
   "eternal-domination-game": () => import("@/components/interactives/EternalDominationGame"),
   "knights-and-knaves": () => import("@/components/interactives/KnightsAndKnavesI"),
+  "three-bank-accounts": () => import("@/components/interactives/ThreeBankAccounts"),
+  "computing-pi": () => import("@/components/interactives/ComputingPi"),
 };