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>

style.css

@@ -178,6 +178,42 @@ header {
     margin-top: 2px;
 }
 
+.stat-wide {
+    grid-column: span 2;
+}
+
+.speed-table {
+    display: flex;
+    flex-wrap: wrap;
+    gap: 0.4rem 0.75rem;
+    margin-top: 0.4rem;
+    justify-content: center;
+}
+
+.speed-item {
+    font-size: 0.85rem;
+    color: #666;
+    white-space: nowrap;
+}
+
+.speed-item.top {
+    font-weight: 700;
+    color: #0f3460;
+}
+
+.speed-bar {
+    display: inline-block;
+    height: 6px;
+    background: #d5dbe5;
+    border-radius: 3px;
+    margin-left: 4px;
+    vertical-align: middle;
+}
+
+.speed-item.top .speed-bar {
+    background: #0f3460;
+}
+
 /* Toolbar */
 .toolbar {
     margin-bottom: 1rem;
@@ -589,6 +625,72 @@ form h3:first-child {
     font-style: italic;
 }
 
+/* Files */
+.file-upload-area {
+    display: flex;
+    align-items: center;
+    flex-wrap: wrap;
+    gap: 0.5rem;
+}
+
+.file-list {
+    display: flex;
+    flex-direction: column;
+    gap: 0.5rem;
+}
+
+.file-item {
+    display: flex;
+    justify-content: space-between;
+    align-items: center;
+    padding: 0.6rem 0.75rem;
+    background: #f8f9fb;
+    border-radius: 8px;
+    border: 1px solid #eee;
+}
+
+.file-info {
+    display: flex;
+    flex-direction: column;
+    min-width: 0;
+}
+
+.file-name {
+    font-size: 0.9rem;
+    font-weight: 500;
+    color: #0f3460;
+    text-decoration: none;
+    overflow: hidden;
+    text-overflow: ellipsis;
+    white-space: nowrap;
+}
+
+.file-name:hover {
+    text-decoration: underline;
+}
+
+.file-meta {
+    font-size: 0.75rem;
+    color: #999;
+}
+
+.file-delete-btn {
+    background: none;
+    border: none;
+    color: #ccc;
+    font-size: 1rem;
+    cursor: pointer;
+    padding: 4px 8px;
+    border-radius: 4px;
+    transition: all 0.15s;
+    flex-shrink: 0;
+}
+
+.file-delete-btn:hover {
+    color: #e74c3c;
+    background: #fef2f2;
+}
+
 /* Responsive */
 @media (max-width: 768px) {
     header {