feat: collapse file picker shortcuts to one line on wide dialogs

[dgageot]

The /attach file picker dialog always rendered its keyboard shortcuts across two rows at the bottom, regardless of how wide the dialog was. On wider terminals this wasted vertical space and made the dialog taller than necessary.

When the dialog is wide enough to fit all shortcuts on a single line, they now render on one row; on narrower dialogs the two-row layout is preserved as a fallback. The dialog height stays stable in both cases, so there is no layout jump when the window is resized across the threshold.

As part of this work, a helpKeysLine helper was extracted in pkg/tui/dialog/base.go so that RenderHelpKeys and HelpKeysWidth share the same formatting logic instead of duplicating it.

[docker-agent]

Assessment: 🟢 APPROVE

The PR cleanly extracts a helpKeysLine helper and uses it to collapse the file picker shortcuts to a single row on wide dialogs. The width threshold logic, split indices (all[:8]/all[8:] correctly splitting 5 key-description pairs as 4+1), and height-stability mechanism (the nil row2 still producing a blank line via a mandatory second AddHelpKeys call) are all correct. No high- or medium-severity bugs were found.