feat(composer): show platform search shortcut

[ewen-poch]

Problem

The composer placeholder does not advertise the existing global-search shortcut, and the displayed key combination needs to match the user's platform.

Proposed change

• Append the global-search shortcut to the composer placeholder.
• Show ⌘K on macOS and Ctrl+K on Windows and Linux.
• Cover all three platform values with an interaction test and align the renderer owner documentation.

Scope and non-goals

This only changes the placeholder copy and its platform-specific presentation. It does not change the existing global-search command, keyboard handling, architecture, or data model.

Acceptance criteria and validation

All checks listed below ran against the final rebased state:

• Platform-specific placeholder behavior -> npm test -- --run src/renderer/src/pages/workspace/ConversationPanel.interaction.test.tsx -> 48 tests passed.
• Renderer and shared type safety -> npm run typecheck -> passed.
• Repository lint rules -> npm run lint -> passed with 0 errors and 18 pre-existing warnings.
• Repository regression suite -> npm test -> 10,072 tests passed and 184 skipped across 690 passed and 15 skipped files.

Uncovered risk: the placeholder was not visually snapshot-tested at every window width; existing responsive text truncation behavior remains unchanged.

Review focus

Please verify the platform mapping and user-facing wording.

[github-actions]

Codex Review

Verdict: mergeable

No actionable findings.

Summary: No concrete merge-blocking defect found. Branch and pull request title checks are valid.