config: validate from at load (empty/invalid from was only caught at send time)

[farangkao]

Problem

While it was my own fault (empty from="") i've run into this behavior myself, the problem is that neomd didn't warn me.
luckily i was able to recover the typed email body using ":recover"
Note: during editing i noticed the empty From field, but i suspected he will simply fallback to my defined email address for the current account selected.

Details:
With from left empty (its default) neomd starts fine, the pre-send screen
shows a blank From: line, and the failure surfaces only after the mail is
written:

✗ build message: invalid From address "": cannot parse address for Message-ID (ensure address format is valid)

The same goes for a malformed from = "xx". extractDomain's comment even
assumes "the From address always comes from validated config.toml accounts" —
but validate() never checked from at all.

Fix

Config.validate() now requires a from on every [[accounts]] entry and
[[senders]] alias, and checks it is a single RFC 5322 address with a
non-empty domain (validFromAddress — the same bar extractDomain applies
at send, so anything that passes load can actually send):

• empty → account "name": from is required (set from to your address, e.g. "Name <you@example.com>")
• malformed ("xx", Name <xx>, multiple addresses) → account "name": from "xx" is not a valid email address

Both surface through the existing config validation: startup error, before
any mail is composed.

Behavior change: configs with an empty from previously loaded (but could
never send) — they now refuse to start, with a message that says exactly what
to set.

Tests

• TestValidate_AccountRequiresFrom, TestValidate_SenderRequiresFrom
• TestValidate_FromFormat (valid/invalid table: "xx", Name <xx>, xx@, multiple addresses)
• TestLoad_EmptyFromIsLoadError (through Load)

Full suite + Hardening suites pass. +157/−0 (code, tests, CHANGELOG entry, AGENTS.md invariant).