cleanup: .is-hidden utility — replaces 3 toggle-display patterns across queue + streamers

Three runtime-toggled elements were each using a slightly different inline display value to flip between visible/hidden:
- vodFilterClearBtn: button, .style.display = '' / 'none'
- btnMergeGroup: button, .style.display = '' / 'none'
- vodBulkBar: div, .style.display = 'flex' / 'none'

Plus each carried an inline style="display:none;" in HTML to start hidden.

Added a single .is-hidden utility class (display:none !important) that hides any element regardless of its natural display type, and:
- HTML now uses class="... is-hidden" instead of style="display:none"
- JS toggles with classList.toggle('is-hidden', !shouldShow) instead of poking at .style.display
- .vod-bulk-bar gets an explicit display:flex in its base rule (was implicit via the JS flip; now declared)

Comment on .vod-bulk-bar animation updated since the trigger is now ".is-hidden removed" rather than "JS sets display:flex".

Net: 3 inline style="display:none" attrs gone from HTML, 4 .style.display assignments gone from TS. Single utility class handles the same job for all three plus any future show/hide toggle.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/index.html

@@ -234,7 +234,7 @@
                 <div class="queue-list" id="queueList"></div>
                 <div class="queue-actions">
                     <button type="button" class="btn btn-start" id="btnStart" onclick="toggleDownload()">Start</button>
-                    <button type="button" class="btn btn-merge-group" id="btnMergeGroup" onclick="createMergeGroupFromSelection()" style="display:none">Merge &amp; Split</button>
+                    <button type="button" class="btn btn-merge-group is-hidden" id="btnMergeGroup" onclick="createMergeGroupFromSelection()">Merge &amp; Split</button>
                     <button type="button" class="btn btn-retry" id="btnRetryFailed" onclick="retryFailedDownloads()" title="Nur fehlgeschlagene Downloads erneut starten">Wiederholen</button>
                     <button type="button" class="btn btn-clear" id="btnClear" onclick="clearCompleted()">Leeren</button>
                 </div>
@@ -263,7 +263,7 @@
                     <div id="streamerProfileHeader" class="streamer-profile-header" style="display:none;"></div>
                     <div class="vod-filter-row">
                         <input type="text" id="vodFilterInput" class="filter-input" placeholder="Filter VODs..." oninput="onVodFilterInput()">
-                        <button type="button" id="vodFilterClearBtn" class="btn-close" onclick="clearVodFilter()" title="Clear filter" style="display:none;">x</button>
+                        <button type="button" id="vodFilterClearBtn" class="btn-close is-hidden" onclick="clearVodFilter()" title="Clear filter">x</button>
                         <label id="vodSortLabel" for="vodSortSelect" class="form-sublabel vod-sort-label">Sort:</label>
                         <select id="vodSortSelect" class="select-compact" onchange="onVodSortChange()">
                             <option value="date_desc">Newest first</option>
@@ -278,7 +278,7 @@
                             <span id="vodHideDownloadedText">Hide downloaded</span>
                         </label>
                     </div>
-                    <div id="vodBulkBar" class="vod-bulk-bar" style="display:none;">
+                    <div id="vodBulkBar" class="vod-bulk-bar is-hidden">
                         <span id="vodBulkCount" class="vod-bulk-count">0 selected</span>
                         <span class="vod-bulk-spacer"></span>
                         <button id="vodBulkAddBtn" class="btn-pill primary" type="button" onclick="bulkAddSelectedVodsToQueue()">+ Queue</button>