fix: notify user when redocking a panel with no song loaded

[carochacs]

_redockPanel() deleted the popups entry and bailed silently when
currentFilename was empty, dropping the popped-out panel's state with
no feedback — unlike the overflow-drop path a few lines down, which
already toasts. Addresses a CodeRabbit review comment on upstream
got-feedBack#23.

[coderabbitai]

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Review failed

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ℹ️ Recent review info

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Run ID: e9fbf360-d4e4-4c6b-90a3-333d0d8390cb

📥 Commits

Reviewing files that changed from the base of the PR and between 749265b and 27c0168.

📒 Files selected for processing (1)

• screen.js

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📝 Walkthrough

Walkthrough

_redockPanel now displays a main-window toast when no song is loaded, replacing the previous silent return.

Changes

Redock feedback

Layer / File(s)	Summary
Notify when redocking has no loaded song screen.js	_redockPanel shows a toast explaining that docking failed because no song is loaded before returning.

Estimated code review effort: 1 (Trivial) | ~2 minutes

Suggested reviewers: byrongamatos, topkoa

Poem

A bunny taps the docking pane,
No song? A toast explains the strain.
No silent hops, the message glows,
Then back the little rabbit goes.

✨ Finishing Touches

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• Commit on current branch

🧪 Generate unit tests (beta)

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• Commit unit tests in branch claude/pr-23-review-j49t5x

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