[KisenaMiyuki]

Why

When a popup opens over CJK or other wide text, parts of its left border go missing.

A wide character takes two columns. If one starts just to the left of the popup, its right half sits under the popup's left border. Clear only resets cells inside the popup, so that wide character stays in the buffer. When Ratatui diffs the frame, it skips the cell hidden under the wide character's right half, so the border character at that row is never sent to the terminal.

What changed

• Added clear_popup_area in src/render/mod.rs. It clears the popup rectangle as before, then replaces any wide character just to the left of the popup that reaches into it with a space.
• Used clear_popup_area in all popups (help, theme, path, and the file, history, and editor pickers).
• The document is redrawn every frame, so the replaced characters come back as soon as the popup closes.

Testing

• Added regression tests in src/tests/render_popup.rs covering the theme and help popups over CJK lines, and checking that the characters return after the popup closes.
• Ran cargo fmt --all -- --check, cargo test, cargo clippy --all-targets --all-features -- -D warnings, and cargo build --release.
• Checked manually in a terminal by opening the theme picker (T) and help (?) over a CJK document.

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Authored by Claude

[RivoLink]

Thanks so much for this PR!