fix(tui): make user_prompt elicitation dialog scrollable

[dgageot]

Fixes #2495.

When `user_prompt` opens an elicitation pop-up with many enum items (or many fields, or a long message), the dialog overflowed the bottom of the terminal with no way to access the clipped content — neither visually nor for keyboard navigation.

What changed

The body region (message + fields, or message + free-form input) is now rendered inside a scrollview, capped at 80% of the terminal height (max 40 rows). Title/separator stay fixed at the top, the help line stays fixed at the bottom.

• The scrollview consumes mouse wheel, scrollbar drag, and PgUp/PgDn/Home/End. Up/Down keep their existing role of cycling enum/boolean selections.
• Auto-scroll keeps the focused field's active line visible:
  • Tab / Shift-Tab brings the new field into view.
  • Up/Down inside an enum/boolean keeps the selected option in view even when the field is taller than the viewport.
  • Submit-with-error focuses and scrolls to the offending field.
• Mouse clicks translate the absolute Y through the current scroll offset before mapping to a field/option, so clicks on visible options still work when scrolled.
• The help line gains a `pgup/pgdn scroll` hint when scrolling is active.
• A single `layout()` method computes geometry (dialog width, viewport, body lines, per-field line offsets) shared by `View()` and `Position()` so layout math lives in one place.
• `buildBody` tracks line offsets incrementally to stay O(N) in the number of fields.

Tests

New unit tests in `pkg/tui/dialog/elicitation_test.go`:

• `TestElicitationDialog_LongEnumScrolls` — 30-option enum on a 20-row terminal needs a scrollbar; selecting an option near the end auto-scrolls it into view.
• `TestElicitationDialog_FieldsBelowFold_AreReachable` — tabbing through 15 fields on an 18-row terminal keeps each focused field visible.
• `TestElicitationDialog_SmallContent_NoScrollbar` — small dialogs do not show a scrollbar.

All existing tests still pass; `golangci-lint run ./...` is clean.

How to test manually

```yaml

/tmp/triage.yaml

version: "2"
agents:
root:
model: openai/gpt-5
description: Triage tester
instruction: |
As soon as the user says anything, call user_prompt EXACTLY ONCE with a
'category' enum field whose 12 options are long incident-triage labels.
toolsets:
- type: user_prompt
```

```sh
./bin/cagent run /tmp/triage.yaml

type any message → dialog appears with a scrollbar; Up/Down/PgUp/PgDn navigate;

all 12 items are reachable; mouse wheel & scrollbar drag work.

```