fix(tui): surface fetchFolders errors

[mvanhorn]

What?

Per-account fetcher.FetchFolders errors are now collected into
FoldersFetchedMsg.Errors and surfaced as a transient overlay in the
TUI (4s, then auto-restore via the existing PluginNotifyMsg pattern).
Closes #1125.

Why?

fetchFoldersCmd spawns one goroutine per account, each calling
fetcher.FetchFolders. The previous code returned immediately on err
and the goroutine vanished without leaving a trace. If an account's
IMAP login was broken, OAuth had expired, or the server was
unreachable, the affected account silently dropped out of the merged
folder list and the user got no signal -- their folder list quietly
missed entries. The reporter described this exactly: "user sees the
folder list silently miss those folders and never gets a notification."

How

File	Change
tui/messages.go	New Errors map[string]error field on FoldersFetchedMsg.
main.go (producer at fetchFoldersCmd)	Capture per-account fetch errors into a sibling map under the existing mutex; pass through to the message.
main.go (consumer at FoldersFetchedMsg case)	When Errors is non-empty, surface a transient overlay listing affected accounts and their errors.

The producer change is a 7-line localized capture: same goroutine, same
mutex, same return-after-error path -- the only difference is that the
err is stored before returning instead of being thrown away.

The consumer surfaces errors using the same pattern the codebase already
uses for PluginNotifyMsg: save m.previousModel, swap m.current to
a tui.NewStatus(...) overlay, fire tea.Tick(4*time.Second, ...) that
returns RestoreViewMsg. After 4 seconds the user is back to where
they were. No new TUI primitives, no new message types beyond the field
on the existing message.

Account display name lookup prefers Email -> Name -> ID so
partially configured accounts still show something meaningful in the
overlay. Errors are sorted alphabetically so the rendered list is
deterministic across restarts (the previous, per-iteration map order
would jitter).

The disjoint property (an account is in either FoldersByAccount or
Errors, never both) is preserved by the existing return after
recording the error.

Verification

make lint     # go fmt ./... + go vet ./...; clean
make test     # all packages pass
go build ./... # clean

I did not add automated tests for the TUI surface itself -- the existing
test suite has no tests for main.go's message handlers (verified via
find . -name '*_test.go'), so introducing the first one felt like
scope creep for a fix PR. Happy to add a coverage test for the
disjoint-by-construction property in fetchFoldersCmd if you'd like;
just point me at the testing pattern you'd want.

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