The multi-account resolver had two bugs that prevented webhook routes
from registering:
1. `accounts.default` was ignored because `resolveLineAccount` short-
circuited the account lookup whenever `accountId` resolved to
`DEFAULT_ACCOUNT_ID`. Credentials placed under
`channels.line.accounts.default` therefore never reached the
gateway, and the default `/line/webhook` route never registered.
2. Named accounts defaulted to disabled when they did not explicitly
set `enabled: true`. A configured second account
(`channels.line.accounts.<name>`) was treated as disabled before
`startAccount` ran, so its webhook never registered either.
Both behaviours diverged from the Telegram plugin, which uses
`baseEnabled && accountEnabled` with each defaulting to
`enabled !== false`. Align the LINE resolver with that idiom and add
seven regression tests covering the credential lookup, the default-
enabled semantics, and channel-level disable propagation.
Peter Steinberger