feat(ui): show relative timestamps on Dashboard and Downloads tables

Converts the Source Health last-check column and the Downloads created-at
column from locale date strings to human-relative times ("6 hours ago").
The absolute timestamp is preserved as a native browser tooltip so full
precision is still one hover away; detail modals keep the absolute form.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/views/Dashboard.vue

@@ -317,7 +317,7 @@
                   <td>
                     <v-chip color="error" size="small">{{ source.error_count }}</v-chip>
                   </td>
-                  <td>{{ formatDate(source.last_check) }}</td>
+                  <td><span :title="formatDate(source.last_check)">{{ source.last_check ? formatRelativeTime(source.last_check) : 'Never' }}</span></td>
                   <td>
                     <v-btn
                       size="small"