Decide primary-correspondent resolution for multi-address To/Cc headers

[Hohnik]

Part of #12

Question

#10's grouping logic resolves the primary correspondent as "the sender (inbound) or recipient (outbound), excluding every address across all connected accounts" — but From/To/Cc are stored as raw header strings that can contain more than one comma-separated address (e.g. a message To two people, only one of which is a connected account).

Decide the exact address-parsing and selection rule for turning a raw To header into "the" primary correspondent when it contains more than one address — including what happens when zero or more than one non-account address remains after exclusion — and record the answer as an update to #10's acceptance criteria.

[Hohnik]

Resolution

• Parsing: use email.utils.getaddresses([header_value]) to split the raw From/To header into individual addresses; lowercase each for comparison against connected accounts' Account.address (also lowercased).
• Exclusion: filter out every address matching a connected account, from whichever header is relevant (From for inbound, To for outbound).
• Exactly one non-account address remains: that's the primary correspondent.
• Zero remain (mail exchanged only between your own connected accounts, e.g. self-forwarding between two accounts): resolves to one global sentinel Contact — address me@mailchat.local, display name "Me" — shared across all account pairs, still split into separate Conversations by subject like any real contact. Not skipped or dropped.
• More than one remains (e.g. an outbound message to two external people): pick the first non-account address in header order (the order getaddresses returns). No fan-out to multiple conversations, no group-chat model — participant-set-aware grouping stays out of scope (already excluded via Spec: MailChat MVP — email-as-chat #7).
• Unselected addresses aren't lost: they remain visible in the raw to_header/cc_header/from_header already stored on Message; future display/search can re-parse on demand. No schema change needed.
• Cc is never consulted for primary-correspondent resolution, in any case.

Folded into #10's acceptance criteria.