refactor: modularize app.js into ES module architecture

Split the monolithic 5393-line app.js into 32 focused ES modules under
modules/ with a thin ~330-line orchestrator. No bundler required — runs
in-browser via import maps.

Module structure:
  core/     — scene, ticker, state, theme, audio
  data/     — gitea, weather, bitcoin, loaders
  terrain/  — stars, clouds, island
  effects/  — matrix-rain, energy-beam, lightning, shockwave, rune-ring, gravity-zones
  panels/   — heatmap, sigil, sovereignty, dual-brain, batcave, earth, agent-board, lora-panel
  portals/  — portal-system, commit-banners
  narrative/ — bookshelves, oath, chat
  utils/    — perlin

All files pass node --check. No new dependencies.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

modules/data/bitcoin.js

@@ -0,0 +1,34 @@
+// modules/data/bitcoin.js — Bitcoin block height polling
+import { state } from '../core/state.js';
+
+const blockHeightDisplay = document.getElementById('block-height-display');
+const blockHeightValue = document.getElementById('block-height-value');
+
+export async function fetchBlockHeight() {
+  try {
+    const res = await fetch('https://blockstream.info/api/blocks/tip/height');
+    if (!res.ok) return;
+    const height = parseInt(await res.text(), 10);
+    if (isNaN(height)) return;
+
+    if (state.lastBlockHeight !== 0 && height !== state.lastBlockHeight) {
+      if (blockHeightDisplay) {
+        blockHeightDisplay.classList.remove('fresh');
+        void blockHeightDisplay.offsetWidth;
+        blockHeightDisplay.classList.add('fresh');
+      }
+      state.starPulseIntensity = 1.0;
+    }
+
+    state.lastBlockHeight = height;
+    state.blockHeight = height;
+    if (blockHeightValue) blockHeightValue.textContent = height.toLocaleString();
+  } catch (_) {
+    // Network unavailable — keep last known value
+  }
+}
+
+export function startBlockPolling() {
+  fetchBlockHeight();
+  setInterval(fetchBlockHeight, 60000);
+}