style(frontend): align dark-mode surfaces with the slate dark palette

The project's dark theme uses the custom slate-based dark-* palette
(928 dark:bg-dark-* / 787 dark:border-dark-* occurrences), but a
minority of surfaces still used Tailwind's neutral gray-* scale, which
has a visibly cooler hue next to slate cards — most noticeably the
filter dropdown panels on the usage, accounts and subscriptions pages.

Migrate the remaining dark:bg-gray-* / dark:border-gray-* (and their
hover: variants) to the dark-* palette across 12 components: filter
dropdowns, chart card borders, menu separators, and small badges.
49 class-string substitutions, no structural or logic changes;
dark:text-gray-* is untouched since gray text on dark surfaces is the
established project-wide convention.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/components/user/dashboard/UserDashboardCharts.vue

@@ -44,7 +44,7 @@
                 </tr>
               </thead>
               <tbody>
-                <tr v-for="model in models" :key="model.model" class="border-t border-gray-100 dark:border-gray-700">
+                <tr v-for="model in models" :key="model.model" class="border-t border-gray-100 dark:border-dark-700">
                   <td class="max-w-[100px] truncate py-1.5 font-medium text-gray-900 dark:text-white" :title="model.model">{{ model.model }}</td>
                   <td class="py-1.5 text-right text-gray-600 dark:text-gray-400">{{ formatNumber(model.requests) }}</td>
                   <td class="py-1.5 text-right text-gray-600 dark:text-gray-400">{{ formatTokens(model.total_tokens) }}</td>