feat: skip-version + addStreamer validation + smart-scheduler tooltip

Three small UX wins.

1. Auto-update: "Skip this version" button on the update modal.
   Stores the dismissed version in localStorage; subsequent automatic
   update-available events for the same version are silenced (banner
   hidden, modal not opened). Manual "Check for updates" overrides the
   skip so the user can change their mind. The flag is cleared once
   the version is actually downloaded so a stale entry never masks a
   future update. Skip button is hidden in the "ready to install"
   state where it would not make sense.

2. addStreamer now validates against Twitch username rules
   (4-25 chars, [a-zA-Z0-9_]). Previously bad input fell through to
   the API and the user saw a silent "streamer not found" message
   instead of being told the input was invalid.

3. Smart Queue Scheduler checkbox got a hover tooltip that explains
   what enabling it actually does ("prefers shorter VODs and older
   queue entries first"). Users were disabling it without knowing
   what they were turning off.

DE + EN locale strings added for all three.

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

src/index.html

@@ -39,6 +39,7 @@
 
             <div class="modal-actions update-modal-actions">
                 <button class="btn-secondary" id="updateModalDismissBtn" type="button" onclick="dismissUpdateModal()">Nein</button>
+                <button class="btn-secondary" id="updateModalSkipBtn" type="button" onclick="skipUpdateVersion()">Diese Version ueberspringen</button>
                 <button class="btn-primary" id="updateModalConfirmBtn" type="button" onclick="confirmUpdateModal()">Ja, herunterladen</button>
             </div>
         </div>