Support uploading FIT files (and zipped exports) alongside GPX

Maptool::Fit reads Garmin's binary FIT format directly rather than
adding a gem — the one Ruby option depends on activesupport, unmaintained
since 2022, for a single-purpose binary parser. Only "record" messages
are read (position + altitude; everything else is skipped by byte
length, never interpreted); field numbers and scale/offset are
Garmin's own (fit-python-sdk's profile.py), and the framing plus all
extracted values were checked byte-for-byte against 5 real device
files from python-fitparse's test suite before this shipped.

A .zip upload (Garmin's bulk/FIT export format) is expanded into its
.gpx/.fit entries via rubyzip — that one's an actual dependency, since
ZIP's own format isn't the kind of narrow, single-purpose parsing FIT
above is.

views/index.erb

@@ -36,15 +36,15 @@
 
 <div id="gpx-panel" class="gpx-panel" hidden>
   <div class="gpx-panel__header">
-    <strong>GPX</strong>
+    <strong>Tracks</strong>
     <div class="gpx-panel__actions">
       <button type="button" class="gpx-panel__action" data-action="new-folder" title="New folder" aria-label="New folder"></button>
-      <button type="button" class="gpx-panel__action" data-action="upload" title="Upload GPX file" aria-label="Upload GPX file"></button>
+      <button type="button" class="gpx-panel__action" data-action="upload" title="Upload GPX/FIT file" aria-label="Upload GPX/FIT file"></button>
       <button type="button" class="gpx-panel__close" aria-label="Close">&times;</button>
     </div>
   </div>
 
-  <input type="file" class="gpx-file-input" accept=".gpx" multiple hidden>
+  <input type="file" class="gpx-file-input" accept=".gpx,.fit,.zip" multiple hidden>
   <p class="gpx-upload__status" hidden></p>
 
   <div class="gpx-tree"></div>