refactor(errors): one helper that logs an error and tells the user in plain words #1761

Problem

User-visible error handling is inconsistent:

• `mapDatabaseError` / `describeDatabaseError` (plain message + remediation hint) are used by 9 files in `core`, and by none in `lib/features`.
• About 46 strings in `lib/features` put the raw exception into the message (`'...$e'`): Redis 18, Mongo 18, workspace 5, connections 5, recovery 4, extensions 4, others.
• The same situation shows a toast on one path, a `TreeLoadError` on another and nothing on a third. Example: editing a connection showed an error, creating one did not (fixed for creation in fix(connections): report and log a failed save of a new connection #1757).

Proposal

• One helper, for example `reportError(context, error, {where, action})`: logs (see the log-file issue), maps through `mapDatabaseError`, shows an error toast with `displayText`.
• `saveNewConnection` (fix(connections): report and log a failed save of a new connection #1757) becomes a user of it.
• Replace the raw `$e` messages in `lib/features`, starting with Redis and Mongo.
• Decide once which surface is used when: toast for an action, `TreeLoadError` for lazy loads, `QueryErrorView` for queries.

Part of the error-handling epic.