Office tab: create, list and edit documents in place, saved in Seafile

- Choose where documents are saved (library + folder picker, like Files); remembered
- New document/spreadsheet/presentation asks for a name and shows the save location
- Documents in that location are listed in the tab; opening one edits it inside the tab
  with the navigation visible and a Back to Office button
- Fix double slash in editor URLs

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

public/style.css

@@ -191,3 +191,6 @@ input[type=search] { width: 100%; min-height: 44px; padding: 0 14px; border-radi
 .row .art { object-fit: cover; flex: none; background: var(--panel2); }
 
 .dot { width: 12px; height: 12px; border-radius: 50%; flex: none; }
+
+/* Editor inside the Office tab: keep the nav, drop the page padding so the editor fills the space */
+body.inedit #view { padding: 0; display: flex; flex-direction: column; }