fix(config): don't panic on a corrupt config, and don't leave keys wo…

[Aporis3674]

Two problems in aether/src/config.rs, both reached through the same code path.

1. A corrupt config panics instead of erroring

config::load returns Result, but the conversion it calls does not:

let wg_priv = base64::…decode(&p.wg_private_key).expect("decode wg private key");
let mut wg_private_key = [0u8; 32];
wg_private_key.copy_from_slice(&wg_priv);   // panics unless len == 32

Both lines panic on input a user can end up with. Reproduced against ee5a5f5:

panicked at src/config.rs:55:24:
  copy_from_slice: source slice length (20) does not match destination slice length (32)

panicked at src/config.rs:46:14:      # a value that isn't base64

How you get there. config::save uses std::fs::write, which truncates in
place — so a process killed mid-save leaves exactly that truncated file, and
every later start panics instead of re-registering. The user has to work out on
their own that deleting aether.toml fixes it. The caller is already prepared
for an error; it cannot catch a panic.

client_id, a few lines below, is length-checked. These two fields just
missed the same guard.

2. The key file is world-readable

aether.toml holds the WireGuard private key, the MASQUE client private key and
the access token. std::fs::write creates it with the process umask — 0644 on
a typical Linux box — and there is no permission handling anywhere in the tree:

$ grep -rn "set_permissions\|PermissionsExt\|0o600\|OpenOptions" aether/src
$

The change

• TryFrom replaces From, and decode_key names the field that is wrong:
  config: wg_private_key decodes to 20 bytes, expected 32
• the config is written to a temp file, chmod 0600, then renamed. That fixes
  the mode and makes the write atomic, so the truncation behind the panic
  can no longer happen in the first place
• five tests: both malformed shapes, an oversized key, the happy path, and the
  resulting file mode

cargo check           Finished dev profile in 55.21s
cargo test config::   5 passed

config.rs is left rustfmt-clean. No behaviour change for a valid config.

---

Generated by Claude Code