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/api-key.html

@@ -0,0 +1,40 @@
+<!-- Login Modal -->
+<div id="api-key-modal" class="modal">
+    <div class="modal-content">
+        <div class="modal-header">
+            <h2 data-i18n="auth.title">🔑 Login to LED Grab</h2>
+            <button class="modal-close-btn" id="modal-close-x-btn" onclick="closeApiKeyModal()" title="Close">&#x2715;</button>
+        </div>
+        <form id="api-key-form" onsubmit="submitApiKey(event)">
+            <div class="modal-body">
+                <p class="modal-description" data-i18n="auth.message">
+                    Please enter your API key to authenticate and access the LED Grab.
+                </p>
+                <div class="form-group">
+                    <div class="label-row">
+                        <label for="api-key-input" data-i18n="auth.label">API Key:</label>
+                        <button type="button" class="hint-toggle" onclick="toggleHint(this)" title="?">?</button>
+                    </div>
+                    <small class="input-hint" style="display:none" data-i18n="auth.hint">Your API key will be stored securely in your browser's local storage.</small>
+                    <div class="password-input-wrapper">
+                        <input
+                            type="password"
+                            id="api-key-input"
+                            data-i18n-placeholder="auth.placeholder"
+                            placeholder="Enter your API key..."
+                            autocomplete="off"
+                        >
+                        <button type="button" class="password-toggle" onclick="togglePasswordVisibility()">
+                            👁️
+                        </button>
+                    </div>
+                </div>
+                <div id="api-key-error" class="error-message" style="display: none;"></div>
+            </div>
+            <div class="modal-footer">
+                <button type="button" class="btn btn-icon btn-secondary" onclick="closeApiKeyModal()" id="modal-cancel-btn" title="Cancel">&#x2715;</button>
+                <button type="submit" class="btn btn-icon btn-primary" title="Login">&#x2713;</button>
+            </div>
+        </form>
+    </div>
+</div>