feat(tabs,app): tab id handles, fixed-width tabs-lift, slim shell + full-height sidebar

Tabs:
- short-handle ID badge per tab (shortest unique conversationId prefix, min 4)
- fixed-width (w-48) tabs with tabs-lift folder borders

Shell (composition root):
- drop the Dispatch title bar; tabs sit at the very top with a 5px gap
- big faded "Dispatch" watermark centered on an empty chat
- collapsible right sidebar (empty shell) spanning full window height: a
  permanently right-pinned hamburger in the tab row toggles it; in-flow push
  that shrinks the whole left column (tabs included) at >=lg, overlay + backdrop
  below lg; open-by-default on wide / closed on narrow
- main is overflow-hidden with a min-w-0 shrink chain; app.css pins
  html/body/#app height + body overflow hidden so the page never overflows

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