Improve mobile viewport sizing and narrow-screen layout behavior

[Copilot]

Mobile layout was too constrained in three places: the repo selector at the top, the right-side chat area on narrow screens, and the overall viewport height when the browser chrome/address bar changed size. This updates the layout so controls use more of the available width and the app tracks the visible viewport more reliably.

• Viewport height handling

  • Switch the top-level app shell to a visible-viewport-aware height strategy (100dvh with 100svh fallback behavior).
  • Remove h-screen usage from the desktop shell so nested panels inherit the actual available height instead of the browser-reported screen height.

• Mobile graph/chat layout

  • Add min-h-0/min-w-0 on the graph, chat, and split-panel containers so flex children can shrink correctly instead of overflowing.
  • Constrain the mobile chat drawer to the visible screen height (90svh) to avoid oversizing when the browser address bar is present.

• Top selector width on mobile

  • Widen the repo selector trigger on small screens so it uses nearly the full content width.
  • Cap the dropdown content width to the viewport to avoid clipping/overflow.

• Regression coverage

  • Add a focused Playwright spec for mobile layout geometry: selector width and viewport-height fit.

<main className="h-[100dvh] min-h-[100svh]">
  <div className="hidden h-full min-h-0 md:flex md:flex-col" id="desktop" />
  <div className="flex h-full min-h-0 flex-col overflow-hidden md:hidden" id="mobile" />
</main>

// mobile selector
<SelectTrigger className="z-10 -mx-2 w-[calc(100%+1rem)] md:mx-0 md:w-full" />

// mobile chat drawer
<DrawerContent className="md:hidden flex min-h-0 flex-col h-[90svh] max-h-[90svh]" />