Split monolithic index.html and style.css for maintainability

- Extract 15 modals and 3 partials from index.html into Jinja2 templates
  (templates/modals/*.html, templates/partials/*.html)
- Split style.css (3,712 lines) into 11 feature-scoped CSS files under
  static/css/ (base, layout, components, cards, modal, calibration,
  dashboard, streams, patterns, profiles, tutorials)
- Switch root route from FileResponse to Jinja2Templates
- Add jinja2 dependency
- Consolidate duplicate @keyframes spin definition
- Browser receives identical assembled HTML — zero JS changes

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

server/src/wled_controller/templates/modals/pp-template.html

@@ -0,0 +1,40 @@
+<!-- Processing Template Modal -->
+<div id="pp-template-modal" class="modal">
+    <div class="modal-content">
+        <div class="modal-header">
+            <h2 id="pp-template-modal-title" data-i18n="postprocessing.add">Add Processing Template</h2>
+            <button class="modal-close-btn" onclick="closePPTemplateModal()" title="Close">&#x2715;</button>
+        </div>
+        <div class="modal-body">
+            <input type="hidden" id="pp-template-id">
+            <form id="pp-template-form">
+                <div class="form-group">
+                    <label for="pp-template-name" data-i18n="postprocessing.name">Template Name:</label>
+                    <input type="text" id="pp-template-name" data-i18n-placeholder="postprocessing.name.placeholder" placeholder="My Processing Template" required>
+                </div>
+
+                <!-- Dynamic filter list -->
+                <div id="pp-filter-list" class="pp-filter-list"></div>
+
+                <!-- Add filter control -->
+                <div class="pp-add-filter-row">
+                    <select id="pp-add-filter-select" class="pp-add-filter-select">
+                        <option value="" data-i18n="filters.select_type">Select filter type...</option>
+                    </select>
+                    <button type="button" class="pp-add-filter-btn" onclick="addFilterFromSelect()" title="Add Filter">+</button>
+                </div>
+
+                <div class="form-group">
+                    <label for="pp-template-description" data-i18n="postprocessing.description_label">Description (optional):</label>
+                    <input type="text" id="pp-template-description" data-i18n-placeholder="postprocessing.description_placeholder" placeholder="Describe this template...">
+                </div>
+
+                <div id="pp-template-error" class="error-message" style="display: none;"></div>
+            </form>
+        </div>
+        <div class="modal-footer">
+            <button class="btn btn-icon btn-secondary" onclick="closePPTemplateModal()" title="Cancel">&#x2715;</button>
+            <button class="btn btn-icon btn-primary" onclick="savePPTemplate()" title="Save">&#x2713;</button>
+        </div>
+    </div>
+</div>