add yank key in large text view overlay

[robince]

No description provided.

[Copilot]

Pull request overview

Adds a y (“yank”) keybinding to the expanded cell reader overlay so users can copy the currently viewed cell value to the clipboard, consistent with the existing table-pane copy behavior.

Changes:

• Add y handling in the expanded reader key handler to copy the current cell’s value.
• Introduce copyReaderCellValue() to copy the value for the reader’s active column/row context.
• Update UI hints/help text and add a test validating clipboard copy + status message while keeping the reader open.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
internal/ui/model.go	Implements reader-overlay y copy behavior and updates bottom bar/footer/help hints accordingly.
internal/ui/model_test.go	Adds a unit test to verify y copies the reader’s current cell value and preserves overlay state.

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