fix: report a broken config file instead of silently using defaults

[VXNCXNX]

Closes #117.

What's broken

A config file that fails to parse is discarded silently. No message, no warning, exit 0, and every customization is gone with no indication why.

A config that is valid TOML but incomplete does it too, which is the easier way to hit this by accident:

$ cat /tmp/partial.toml
no_hint = true

$ jnv --config /tmp/partial.toml
(defaults, no mention that the file was ignored)

The error was there all along, just thrown away:

TOML parse error at line 1, column 1
  |
1 | no_hint = true
  | ^
missing field `reactivity_control`

The cause

unwrap_or_else(|_e| ...) in main swallows the error and substitutes DEFAULT_CONFIG.

The fix

Propagate it. Now:

$ jnv --config /tmp/partial.toml
Error: TOML parse error at line 1, column 1
  |
1 | no_hint = true
  | ^
missing field `reactivity_control`

$ echo $?
1

Why failing is right here, rather than warning and continuing

ensure_file_exists already creates the file with the defaults when it is absent, for both the explicit --config path and the default location. So "no config" is handled before this point and never reaches it.

That means anything failing here is a file that exists and is broken, which is worth stopping for. A user who passes --config and gets defaults instead has silently lost their settings.

Tell me if you would rather warn and continue and I will change it.

Verification

A valid config still loads: running with default.toml gets past config loading to terminal setup as before.

cargo fmt --check, cargo clippy --all-targets -- -D warnings, cargo test and cargo build all pass, matching the five steps in ci.yml.

No test added, because the repo currently has no test target at all and cargo test reports 0 tests. Adding a harness felt like a bigger change than the fix. Happy to add one if you want the infrastructure.

[ynqa]

@VXNCXNX Thanks, LGTM :)