fix(dm): stop showing "Network error: cancelled" on message send

[Adron]

Problem

Sending a DM surfaced a red "Network error: cancelled" banner even though the message actually sent (see the reported screenshot). Root cause: a cancelled URLSession request (URLError(.cancelled), thrown when a SwiftUI .task is torn down on navigation/teardown) was flattened at the APIClient boundary into APIError.transport(message: "cancelled") and rendered as an error.

Two design defects turned a benign cancellation into a persistent banner:

1. Cancellation treated as a real error — load()/send() catch every error and store it in the visible error; the .cancelled code was already lost at APIClient.swift:210.
2. Sticky error — only a successful load()/send() cleared it; the poll loop that keeps the thread live swallowed errors and never cleared it.

Fixes

• APIClient.performOnce — normalise URLError(.cancelled) / CancellationError to CancellationError instead of laundering it into a transport error. Genuine transport failures (offline, DNS, TLS, timeout) are unchanged.
• View models (DMThreadViewModel.load()/send(), DirectMessagesListViewModel.load()/loadMore(), NewMessageViewModel.send()) — ignore CancellationError instead of pinning it to error.
• DMThreadViewModel.pollOnce() — clear a stale error on a successful poll so a one-off cancellation self-heals.

Tests (all passing)

• InterlinedKit APIClientTests — a cancelled request throws CancellationError, never APIError.transport; existing transport-error test still passes.
• App DMThreadViewModelTests — cancelled load leaves error nil / not-loaded; cancelled send restores the draft without a banner; a successful poll clears a stale error.

Verified: InterlinedKit APIClientTests 11/11; App DMThreadViewModelTests 12/12, DirectMessagesListViewModelTests 11/11, NewMessageViewModelTests 8/8 — 0 failures.

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