fix(ui): prompt to quit on ctrl+d

[officialasishkumar]

Summary

Treat ctrl+d like EOF in the editor when the prompt is empty by opening the existing quit confirmation dialog.

Problem

ctrl+d is a common REPL EOF shortcut, but Crush currently ignores it in the editor unless the user types quit or uses ctrl+c.

Before

1. Start Crush and focus the prompt editor.
2. Leave the prompt empty.
3. Press ctrl+d.
4. Nothing happens.

After

1. Pressing ctrl+d with an empty editor opens the existing quit dialog.
2. Existing compact-mode ctrl+d details behavior is unchanged outside the empty-editor EOF case.
3. ctrl+d does not prompt to quit when there is typed input or pending attachments.

Changes

• Add a narrow EOF check in key handling for ctrl+d while the editor is focused.
• Reuse the existing quit confirmation dialog instead of adding a new quit path.
• Add regression tests for the empty-editor case and guards for typed input and attachments.

Test Plan

• go test ./internal/ui/model

Closes #2428

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