TUI Calendar: Enter does nothing on a highlighted event (Day/Week), though the help bar shows "enter: open"

### What happens

In `hey tui`, Calendar section, Day or Week view: highlight an event with the arrow keys and press **Enter**. Nothing happens.

The context help bar shown while the content row is focused advertises `enter open` (from the generic content-row bindings in `internal/tui/tui.go` `updateHelpBindings`, ~line 743), the same hint that opens a thread in the Mail list. In the calendar Day/Week view that hint is dead — there is no key that opens/views the highlighted event.

### Expected

Enter on a highlighted event opens it (the same result as `e` today), consistent with the help bar and with Mail, where Enter opens the selected row.

### Actual

`internal/tui/calendar.go`:

- `handleContentKey` (~line 1058) has no `"enter"` case.
- `handleArrowKey` (~line 1133) only handles `"enter"` in **Year** view (`enterYearCell`, stepping into a day cell). In `viewDay` / `viewWeek` it returns `handled == false`.
- So Enter falls through to `v.contentVP.Update(msg)` (viewport scroll) and is a no-op.

The only way to open/inspect an event is `e` (opens the edit form, `eventFormEdit`). There is no read-only "view" mode — `eventFormMode` is just `eventFormCreate` / `eventFormEdit`.

### Suggested fix

In `handleContentKey` / `handleArrowKey`, for `viewDay` and `viewWeek`, map `enter` to open the selected recording when there is one:

```go
case "enter":
if event, ok := v.selectedRecording(); ok {
return v.startEventForm(eventFormEdit, event), true
}
```

Year view keeps its two-stage behavior (Enter steps into a cell; once `inYearCell`, Enter could then open the selected event). Happy to send a PR.

### Environment

- hey-cli 1.4.3 (Linux, installed via mise)

Start in internal/tui/calendar.go by reading handleContentKey and handleArrowKey, then run hey tui and reproduce the issue in Calendar Day and Week views. Done means Enter on a highlighted event opens the existing edit form like e, while Year view keeps its current behavior.

Written by the indexing model from the issue text.