Show platform-specific shortcut modifiers

[WilliamWang1721]

Result / 用户结果

Shortcut labels now use the running platform's modifier names: macOS shows Command and Option, Windows shows Win and Alt, and Linux shows Super and Alt. The read-only numbered-tab row also includes Command+1..9 on macOS. No issue link was provided.

Design / 设计边界

• Responsibility and affected modules: nebula_app/src/display/keymap.rs owns shared shortcut labels used by both settings shells and shortcut hints.
• Why this belongs here; interfaces that remain unchanged: formatting stays with the existing shared keymap model; canonical stored combos and GPUI binding semantics remain unchanged.
• Dependency, data-format, threading, or lifetime changes: none.
• Compatibility and migration/fallback behavior: existing keybind values remain readable and unchanged; Windows retains Win/Alt labels.

Evidence / 验证依据

• git diff --check: passed.
• python3 scripts/check_architecture.py --base 9dc058d12765893553d5fc7a2c37c870c96168b0: reports two unchanged base-file limits (display/mod.rs 11067/11064; display/settings.rs 8407/8406). This PR changes only display/keymap.rs.
• Added a regression test for platform labels and canonical combo preservation; tests were not run locally because cargo is unavailable.
• Native UI screenshots, keyboard interaction, and DPI review were not run locally.
• No hot-path behavior was changed beyond shortcut label formatting.

Required Review / 必须确认

• I followed CONTRIBUTING.md, docs/architecture.md, and docs/project-constraints.md.
• I split responsibilities, not arbitrary line ranges; no duplicate behavior authority was added.
• python3 scripts/check_architecture.py --base <PR-base-commit> passes; budgets were not inflated to fit the change. (The checker reports the two unchanged base-file limits listed above.)
• Tests cover success and failure; platform/feature coverage limitations are stated. (A platform-label regression test was added but not run locally.)
• New messages use typed i18n IDs and matching placeholders; untranslated content has an explicit fallback. (No catalog message was added or changed.)
• Governance changes include a counterexample, corrected contract, tests, and a maintainer-reviewed decision. (No governance change.)

UI Review Screenshots

Current-head evidence for 0319fb7534d287bd4cbd41431d99251436e7bdda. UI review workflow run

Windows — success

Raw evidence: Windows originals

macOS — success

Raw evidence: macOS originals