fix(ui): show empty panes as a centered panel

[noyobo]

What & why

Empty panes were a single faint sentence, which read as a placeholder. This adds one centered panel (icon well, title, one line of why) and uses it wherever a whole pane was blank: a whitespace-only diff (including the staged case from #38), an empty file, a binary file, a missing image, no diff, a file with no history, an empty graph, a commit with no files, a welcome search with no matches, and an empty rebase plan. Filter misses and the command palette stay one line, because those are not a pane.

Screenshots

Checklist

• pnpm typecheck && pnpm test pass
• cargo fmt --check && cargo clippy --all-targets -- -D warnings && cargo test pass (if Rust changed)
• New engine functions have integration tests in tests/git_engine.rs
• Mutating operations go through the undo tracked() wrapper where undoable
• Colors use design tokens only (no hex in components)
• CLAUDE.md updated if conventions/architecture changed
• Commits carry no trailers from coding tools (Co-authored-by: <tool>, Generated with …)

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