a11y: aria-label on the 3 icon-only buttons in the main shell

Three icon-only buttons in the always-visible UI had no programmatic accessible name:
- the "+" add-streamer button (no id, no title, no aria-label — entire button was unnamed)
- the bulk-remove X button next to the Streamer section title (had a localized title, but title is not a reliable accessible-name source — many screen readers don't expose it)
- the VOD-filter clear X button above the VOD grid (same — title-only)

For all three the visible text is just a glyph ("+", "x"), so the accessible name has to come from somewhere else. Added:
- new locale key static.streamerAddAriaLabel ("Streamer hinzufuegen" / "Add streamer") and an id on the "+" button so renderer-texts can localize it
- new setAriaLabel(id, value) helper in renderer-texts mirroring the existing setTitle/setPlaceholder pattern
- aria-label calls for all three buttons, in addition to the existing title (so the tooltip stays for sighted users)

The two existing X buttons reuse their existing title strings as aria-label — no new translation work, just exposing the already-present text via the right ARIA attribute.

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

src/index.html

@@ -248,7 +248,7 @@
                 <div class="header-actions">
                     <div class="header-search">
                         <input type="text" id="newStreamer" placeholder="Streamer hinzufugen..." onkeypress="if(event.key==='Enter')addStreamer()">
-                        <button onclick="addStreamer()">+</button>
+                        <button id="btnAddStreamer" type="button" onclick="addStreamer()" aria-label="Add streamer" title="Add streamer">+</button>
                     </div>
                     <button class="btn-icon" onclick="refreshVODs()">
                         <svg width="16" height="16" viewBox="0 0 24 24" fill="currentColor"><path d="M17.65 6.35C16.2 4.9 14.21 4 12 4c-4.42 0-7.99 3.58-7.99 8s3.57 8 7.99 8c3.73 0 6.84-2.55 7.73-6h-2.08c-.82 2.33-3.04 4-5.65 4-3.31 0-6-2.69-6-6s2.69-6 6-6c1.66 0 3.14.69 4.22 1.78L13 11h7V4l-2.35 2.35z"/></svg>