Add file uploads and speed distribution chart

- File upload/download/delete per customer (max 20MB, stored in data/files/)
- Files section shown in customer detail modal
- Speed distribution chart replaces single "top speed" stat
- Bar chart shows all speeds with count, top speed bolded
- Customer delete also cleans up associated files
- data/files/ added to .gitignore

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

index.html

@@ -58,10 +58,9 @@
                     <div class="stat-value" id="stat-top-zip">-</div>
                     <div class="stat-sub" id="stat-top-zip-detail"></div>
                 </div>
-                <div class="stat-card stat-trivia">
-                    <div class="stat-label">Suosituin nopeus</div>
-                    <div class="stat-value" id="stat-top-speed">-</div>
-                    <div class="stat-sub" id="stat-top-speed-detail"></div>
+                <div class="stat-card stat-trivia stat-wide">
+                    <div class="stat-label">Nopeudet</div>
+                    <div id="stat-speed-table" class="speed-table"></div>
                 </div>
                 <div class="stat-card stat-trivia">
                     <div class="stat-label">Keskihinta / kk</div>