Fix blank page from stale cached assets; surface errors

A browser or CDN holding an assets/app.js from before the shell's element
IDs were renamed runs it against the newer index.php, where it grabs
btn-logout/btn-rescan/nav-user/nav-username/app — all now null — and throws
before it can unhide #app-shell. The result is a blank page whose only clue
is a TypeError in the console.

- index.php: asset() stamps local css/js with the file mtime, so a deploy is
  a new URL and a stale copy can't be paired with a newer shell.
- index.php: install an error handler ahead of every other script (capture
  phase, so failed resource loads are caught too) that paints uncaught
  errors, rejections and load failures into a #boot-error panel. It works
  even when app.js itself is what failed, which is this case.
- includes/db.php: add a register_shutdown_function fatal handler, since
  set_exception_handler never sees parse errors, exhausted memory or a
  failed require — those were ending requests with an empty body. Both
  handlers flush partial output and answer in JSON; display_errors stays off
  so a warning can't corrupt a JSON body.
- New optional DEBUG config flag adds exception type, file:line and trace to
  those responses. config.php is gitignored, so installs without the setting
  default to false.

Claude-Session: https://claude.ai/code/session_014oyi9DMdhALrGz989MNNZJ

assets/style.css

@@ -1166,3 +1166,22 @@ button, input { font-family: inherit; font-size: inherit; }
   * { animation-duration: .001ms !important; animation-iteration-count: 1 !important; transition-duration: .001ms !important; }
   .is-scanning .scan-spin, .is-scanning .scan-mobile svg { animation: spin 1.4s linear infinite !important; }
 }
+
+/* Boot failure report (index.php installs the handler that creates this).
+   Deliberately self-contained — no custom properties — because the whole
+   point is to stay readable when the app never started. */
+#boot-error {
+  position: fixed;
+  inset: 16px;
+  z-index: 9999;
+  overflow: auto;
+  margin: 0;
+  padding: 20px;
+  border: 1px solid #B84A3C;
+  border-radius: 10px;
+  background: #FFF6F4;
+  color: #5A1E16;
+  font: 500 13px/1.6 ui-monospace, SFMono-Regular, Menlo, Consolas, monospace;
+  white-space: pre-wrap;
+  word-break: break-word;
+}