fix(ui): prevent status-bar panic on multi-byte error messages

[svaiml]

Problem

render_status_bar truncates an overflowing status line with a raw byte slice:

format!("{}...", &status[..center_width.saturating_sub(3)])

When the status line holds an error message, that message can contain multi-byte
UTF-8 characters — GitHub API error bodies, GraphQL error text, and org/repo
names all routinely do. If center_width - 3 falls inside such a character,
the slice panics with byte index N is not a char boundary. Because this runs
inside the ratatui draw closure, the panic tears down the whole TUI and leaves
the terminal in a broken raw state.

The truncation branch triggers whenever status.len() > center_width, i.e. on a
sufficiently narrow terminal (or a long error) — so a single non-ASCII byte in an
error message is enough to crash the app on the next frame.

Fix

Extract the truncation into a small truncate_status helper that walks the cut
point back to the nearest UTF-8 character boundary before appending the ellipsis.
Behaviour is byte-for-byte identical for ASCII status lines; multi-byte lines are
now cut safely instead of panicking.

Tests

Added unit tests for:

• no truncation when the status fits,
• ASCII truncation with the ellipsis,
• a cut that lands inside a multi-byte character (the previous panic),
• a sweep over every width of a multi-byte-dense string to guarantee the path is
  panic-free.

cargo build --all-targets, cargo test, cargo clippy --all-targets -- -D warnings, and cargo fmt --check all pass.