fix(ui): prevent error and modal overflow

[sebasv]

Summary

Long errors were silently clipped to the modal's single-line slot, hiding the actionable tail (e.g. No such file or directory (os error 2) rendered as just No such file). Modals also assumed a generously sized terminal and broke their layout when shrunk. This PR introduces shared wrap/truncate helpers and applies them across every modal that surfaces an error or a user-supplied path/branch/repo name.

Changes

• Wrap & ellipsize errors. New wrap_message(msg, width, max_lines) in ui/mod.rs honors embedded \n (anyhow's {:#} formatting), word-wraps, hard-breaks paths longer than the line, and ellipsizes the last shown line when content gets dropped (cap: 3 lines). Used by add_repo (both variants), confirm::render_remove_worktree, and discovered. Modal heights grow to match wrapped error length so we never push the slot past the border.
• Continuation indent. Wrapped error continuations use a 4-col indent so they line up under the first character of the message rather than under the ! prefix.
• Truncate long titles & body strings. Modal titles, the Worktree: line, and the unmerged-branch warning all run truncate_to_width against the available budget. Long branch names can no longer eat the closing border corner.
• Min-size guard. Below 50×12, ui::render paints a yellow Terminal too small (need at least 50×12) placeholder instead of letting the sidebar+main+modal layout collapse.
• Shared truncate helper. Promoted sidebar's private truncate to pub fn truncate_to_width in ui/mod.rs; sidebar reuses it.

Test plan

• cargo test — 153 passed (12 new: wrap helpers + tiny-terminal guard + long-error wrap snapshot for AddRepo).
• cargo clippy --all-targets -- -D warnings clean.
• cargo fmt --all -- --check clean.
• Updated snapshots for AddRepo modal show the full ... No such file or directory (os error 2) wrapped onto two lines, with the modal height growing by one row.
• Manual smoke: trigger an Add Repo failure with a very long path, resize terminal below 50×12, verify graceful placeholder and recovery on resize.

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