cleanup: streamerListFilter + btnStreamerBulkRemove use .is-hidden

Continuing the .is-hidden migration from 4.6.134 — two more sidebar-shell elements were doing the same .style.display = '' / 'none' dance:

- streamerListFilter input (compact filter that only appears once the streamer list crosses STREAMER_FILTER_THRESHOLD)
- btnStreamerBulkRemove button (X bulk-remove button, same threshold gate, plus a separate hide path inside the no-streamers branch)

Both started with style="display:none" in HTML and toggled via .style.display in renderer-streamers.ts. Now use the shared .is-hidden class — HTML drops the inline style, JS uses classList.toggle/add.

3 more .style.display assignments + 2 inline display:none HTML attrs gone, identical behaviour.

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

src/index.html

@@ -221,9 +221,9 @@
                     <span id="streamerSectionTitleText">Streamer</span>
                     <span id="streamerSectionCounter" class="streamer-section-counter"></span>
                 </span>
-                <button id="btnStreamerBulkRemove" class="btn-close" type="button" onclick="bulkRemoveStreamers()" title="Bulk remove" style="display:none;">x</button>
+                <button id="btnStreamerBulkRemove" class="btn-close is-hidden" type="button" onclick="bulkRemoveStreamers()" title="Bulk remove">x</button>
             </div>
-            <input type="text" id="streamerListFilter" class="filter-input compact" placeholder="Filter..." oninput="onStreamerListFilterChange()" style="display:none;">
+            <input type="text" id="streamerListFilter" class="filter-input compact is-hidden" placeholder="Filter..." oninput="onStreamerListFilterChange()">
             <div class="streamers" id="streamerList"></div>
 
             <div class="queue-section">