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/index.html

@@ -32,6 +32,8 @@
       <p class="admin-copy">Server-wide settings, shared by every dashboard. Saving re-initialises the
         module immediately — open tiles reconnect on their own. Per-tile options (filters, text size)
         live in each tile's gear menu instead.</p>
+      <p class="admin-copy admin-storage" id="storage-note" hidden>Saved on the server at
+        <code id="storage-path"></code> — outside the app folder, so updating ProdDash keeps them.</p>
       <div id="modules-list"></div>
     </section>