Add dark mode, simplify nav, and UI polish

- Dark mode via prefers-color-scheme: pure CSS, no JS, follows OS setting
- CSS custom properties for all colors; dark palette redefines variables
- Nav: brand is now a home link, Dashboard link removed, nowrap keeps
  it single-line on mobile; compact padding at 640px breakpoint
- Battery health % uses CSS classes (health-good/warn/bad) instead of
  inline Jinja colors — readable in both light and dark modes
- Stat card counts use CSS color variables for dark mode support
- Fix duplicate display:none on bulk toolbar
- input[type=date] added to themed input selector

templates/battery_detail.html

@@ -46,11 +46,11 @@
         {{ battery.capacity_mah }} mAh
         {% if battery.tested_capacity_mah %}
           {% set pct = (battery.tested_capacity_mah / battery.capacity_mah * 100)|round|int %}
-          {% if pct >= 80 %}{% set health_color = "#166534" %}
-          {% elif pct >= 60 %}{% set health_color = "#92400e" %}
-          {% else %}{% set health_color = "#991b1b" %}
+          {% if pct >= 80 %}{% set health_class = "health-good" %}
+          {% elif pct >= 60 %}{% set health_class = "health-warn" %}
+          {% else %}{% set health_class = "health-bad" %}
           {% endif %}
-          &rarr; tested <strong style="color:{{ health_color }};">{{ battery.tested_capacity_mah }} mAh ({{ pct }}%)</strong>
+          &rarr; tested <strong class="{{ health_class }}">{{ battery.tested_capacity_mah }} mAh ({{ pct }}%)</strong>
           {% if battery.tested_date %}<span class="text-muted">on {{ battery.tested_date }}</span>{% endif %}
         {% endif %}
       </td>