fix(ui): add ic-btn-reset CSS + auto-convert auth.json to cookies.txt

- layout.css: add button.ic-btn-reset rule so all icon buttons
  (bell, back, close, retry, etc.) get proper background:none reset
  instead of browser-default white/grey appearance in dark mode
- instagram-extractor.ts: auto-convert secrets/auth.json
  (Playwright storage format) to Netscape cookies.txt at runtime
  whenever auth.json is newer; ensures sessionid and all Instagram
  session cookies are passed to yt-dlp, fixing empty media response

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/routes/layout.css

@@ -99,7 +99,8 @@ html, body {
 }
 .ic-scroll::-webkit-scrollbar { display: none; }
 
-button.ic-btn {
+button.ic-btn,
+button.ic-btn-reset {
   background: none;
   border: 0;
   padding: 0;