fix(picker): format fzf preview entries to list width

[hermandavid]

Issue

PR #399 fixed the fzf-lua session picker preview gutter, but the width handling for preview mode still used the full picker width for session rows. In practice, that pushed the datetime column off the visible list pane and truncated session rows down to the title and ellipsis.

Solution

Make preview-aware width handling happen where picker item width is actually determined. The list width is now computed before the picker callback is wrapped, so fzf preview entries use the visible list pane width while the picker window itself still uses the full width.

Changes

• Move the fzf preview width adjustment into base_picker.pick(), where item
  width is passed into the picker callback.
• Remove the ineffective @opts.width mutation from fzf_ui(), which happened
  too late to affect the rendered session rows.
• Add a regression test that verifies preview-mode entries use the list-pane
  width while fzf still receives the full window width.

[hermandavid]

This is a fix for a bug introduced in #399