client/web, tsweb/compserve: serve index.html for the root path

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GET / in the client/web assets handler normalized to an empty fs
path. The empty name is invalid, and file systems wrapped in fs.Sub
reject it with fs.ErrInvalid instead of fs.ErrNotExist. The
index.html fallback only triggers on fs.ErrNotExist, so it never ran
and the request failed with a 500 "internal error". This broke the
corp smoke test that curls http://100.100.100.100/.

Normalize the empty path to index.html before serving. Also make
compserve.ServeFile return fs.ErrNotExist for invalid paths, per its
documented contract, instead of leaking fs.ErrInvalid.

Both layers have regression tests verified to fail without the fix.

Updates tailscale/corp#20099
Updates #12170

Signed-off-by: James Tucker <james@tailscale.com>

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500 Internal Server Error

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