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/capture-template.html

@@ -0,0 +1,45 @@
+<!-- Template Modal -->
+<div id="template-modal" class="modal">
+    <div class="modal-content">
+        <div class="modal-header">
+            <h2 id="template-modal-title" data-i18n="templates.add">Add Capture Template</h2>
+            <button class="modal-close-btn" onclick="closeTemplateModal()" title="Close">&#x2715;</button>
+        </div>
+        <div class="modal-body">
+            <input type="hidden" id="template-id">
+            <form id="template-form">
+                <div class="form-group">
+                    <label for="template-name" data-i18n="templates.name">Template Name:</label>
+                    <input type="text" id="template-name" data-i18n-placeholder="templates.name.placeholder" placeholder="My Custom Template" required>
+                </div>
+
+                <div class="form-group">
+                    <label for="template-description" data-i18n="templates.description.label">Description (optional):</label>
+                    <input type="text" id="template-description" data-i18n-placeholder="templates.description.placeholder" placeholder="Describe this template..." maxlength="500">
+                </div>
+
+                <div class="form-group">
+                    <div class="label-row">
+                        <label for="template-engine" data-i18n="templates.engine">Capture Engine:</label>
+                        <button type="button" class="hint-toggle" onclick="toggleHint(this)" title="?">?</button>
+                    </div>
+                    <small class="input-hint" style="display:none" data-i18n="templates.engine.hint">Select the screen capture technology to use</small>
+                    <select id="template-engine" onchange="onEngineChange()" required>
+                    </select>
+                    <small id="engine-availability-hint" class="form-hint" style="display: none;"></small>
+                </div>
+
+                <div id="engine-config-section" style="display: none;">
+                    <h3 data-i18n="templates.config">Configuration</h3>
+                    <div id="engine-config-fields"></div>
+                </div>
+
+                <div id="template-error" class="error-message" style="display: none;"></div>
+            </form>
+        </div>
+        <div class="modal-footer">
+            <button class="btn btn-icon btn-secondary" onclick="closeTemplateModal()" title="Cancel">&#x2715;</button>
+            <button class="btn btn-icon btn-primary" onclick="saveTemplate()" title="Save">&#x2713;</button>
+        </div>
+    </div>
+</div>