feat(ssh): collapse host groups in the settings library

[Aschenbath]

Result / 用户结果

Fixes #480. The SSH host page now groups hosts under collapsible headings with counts. Search temporarily reveals matching hosts; clearing the query restores the view's fold state. An entirely ungrouped library keeps its existing flat layout.

Design / 设计边界

• Grouping is a presentation projection of the existing filtered host snapshot. Groups follow their first host and preserve host order within each group. The existing virtual list remains bounded to eight visible rows.
• Fold state belongs to the settings view and is not persisted. No new dependency, host-store format, credential behavior, or connection path.
• Headings use GPUI button roles, full-row hit targets, visible keyboard focus, Tab navigation, Enter/Space activation, and accessible expanded state. Labels align with the list and truncate independently of counts. Search exposes non-interactive headings. English and Chinese action descriptions use typed IDs; other languages fall back to English.
• English and Chinese READMEs explain ordering, search, and lifetime behavior.

Evidence / 验证依据

• Windows host-library suite: 5 passed. Covers actual heading hit areas, Tab/Enter/Space, search reveal/restore, ungrouped compatibility, stable ordering and counts, and existing host actions.
• Native Windows fixture verified at 192 DPI / 200%: Nord expanded groups and Paper with the home group collapsed. A native pointer click on the real heading hides its three hosts while retaining the count and other groups. Window-owned capture contains synthetic hosts only. Other DPI values, long translated labels and Linux/macOS native visual behavior remain unverified locally.
• Product cargo check --locked -p nebula --bin pebrel --features gpui-shell --tests: passed.
• Internationalization contract: 24 passed, 1 manual benchmark ignored. Settings: 97 passed. File-line contract: 2 passed. Architecture check against 33479963: passed; checker regression suite: 54 passed.
• Changed Rust files formatted; git diff --check passed. Full-workspace formatting and Linux/macOS native tests were not run locally; CI results must be evaluated separately.

Required Review / 必须确认

• Followed contribution, architecture and project constraints.
• Responsibilities remain separate; no duplicate persistence authority.
• Architecture check passed without raised budgets.
• Regression coverage and verification limits stated.
• Typed i18n IDs and matching placeholders, with English fallback.
• Governance changes: not applicable.