fix: support Windows UNC paths in explorer and produced-file resolution

[LuckinOvaltine]

Problem

On Windows, when a session's working directory is a UNC network path
(e.g. \\server\share\project), the sidebar explorer fails to list the
directory with "... is not an absolute path", and the produced-file
"open in sidebar" feature mis-resolves UNC paths.

requireAbsolute() only accepted /-prefixed and drive-letter (C:\)
paths; resolveSidebarPath() had the same gap in the client half.

Changes

• src/fs-tree.ts: use Node's path.isAbsolute() (which returns true
  for UNC paths on win32) in requireAbsolute().
• src/client/produced-files.ts: treat \\-prefixed paths as absolute
  in resolveSidebarPath().
• tests/unit.spec.ts: add win32 cases covering UNC paths for both
  helpers.

How I verified

• pnpm typecheck passes.
• pnpm build passes (tsc + tsdown).
• New UNC test cases pass. The remaining pnpm test failures on a local
  Windows machine are pre-existing platform issues (CRLF line endings,
  /bin/bash vs powershell.exe, drive-letter resolution) and are
  unchanged by this PR.

Risks / Notes

• No behavior change for POSIX or drive-letter absolute paths.
• UNC detection is guarded to the win32 platform in tests.

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