Say which config client control could not find a profile in

[andrewchen5678]

A bare flextunnel client control under the systemd template layout (docs/systemd.md: one <instance>.toml per unit, no client.toml) failed with:

Error: The profile has no server node id (set server_node_id in the config or pass -n).

which blames a profile that was never found — the default config does not exist at all in that layout.

What changed

• client control now reports the three cases separately:
  • -c <file> with no server_node_id — names that file.
  • default client.toml with no server_node_id — names it, and mentions -c for another profile.
  • no config file at all — says so and lists the *.toml profiles found in ~/.config/flextunnel (exactly the set of systemd instances):

    Error: There is no client config at /home/user/.config/flextunnel/client.toml, so `client control` has no profile to attach to. Pass the profile's config with -c <file>, or attach by server id with -n <server EndpointId>.
    Profiles in /home/user/.config/flextunnel: aws.toml, macintel.toml
    

• Dropped the .context() that wrapped genuine read/parse failures with "client control needs a profile", hiding the real reason.
• Added config::default_client_config_path() so the default path can be named in errors.
• docs/systemd.md: note that -c is not optional under the template layout.
• Version bump to 0.0.77.

Testing

cargo clippy --workspace --all-targets clean; cargo test --workspace passes, including three new tui tests.

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https://claude.ai/code/session_017qaKrfofmiVWdkXnyjubXF