Keep module config and layouts in a per-machine data directory

Connection settings, API keys, enabled flags and named layouts were written
to config/ inside the checkout, so a re-clone or reset from main started the
server with nothing configured. They now live in a data directory outside
the app folder:

  Windows  %APPDATA%\ProdDash
  macOS    ~/Library/Application Support/ProdDash
  other    $XDG_CONFIG_HOME/proddash, else ~/.config/proddash

PRODDASH_DATA_DIR overrides the location (a relative path resolves against
the app folder, so "config" restores the old spot). The first start after
upgrading copies an existing config/modules.json and config/layouts/ into
the data directory, so nothing has to be re-entered. If the directory cannot
be created or written, the server warns and falls back to config/ rather
than refusing to start. A proddash.json in the data directory overrides the
checked-in defaults (port, adminPasscode).

The startup banner prints the directory in use, /api/admin/state exposes it,
and the admin page shows it under the Modules heading. Admin config and
layout writes that fail on disk now report the path instead of a generic
server error. README, MODULE-GUIDE and CLAUDE.md updated.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RNYUv9EyrGSFmCnmH7WYKH

public/admin/admin.js

@@ -382,6 +382,11 @@ async function loadState() {
   }
   loginCard.hidden = true;
   modulesSection.hidden = false;
+  const storageNote = document.getElementById('storage-note');
+  if (state.dataDir) {
+    document.getElementById('storage-path').textContent = state.dataDir;
+    storageNote.hidden = false;
+  }
   modulesList.innerHTML = '';
   liveEls.clear();
   for (const mod of state.modules || []) {