feat(desktop): add @everyone channel mention

[EmpireOperating]

Summary

• Adds an @everyone autocomplete option for channel conversations.
• Expands it client-side into the current channel's other uniquely named members.
• Preserves normal individual, team, and persona mention behavior.

Safety

• Excludes the current user.
• Suppresses @everyone when a member has no display name or display names collide.
• Sends ordinary individual mention tags rather than a special server-side broadcast primitive.

Verification

• pnpm build:e2e
• node --import ./test-loader.mjs --experimental-strip-types --test src/features/messages/lib/mentionCandidates.test.mjs — 5 passed
• Focused Playwright coverage for channel-member expansion passed.

Note: a full mentions.spec.ts run reached 33 passing tests before the local Playwright web server stopped accepting connections. The focused regression test and baseline comparison both passed.

[EmpireOperating]

Closing as superseded by the more complete active channel-wide mention work in #6050 and #4777. Those efforts cover the same product need across the Desktop, CLI, and SDK surfaces, so keeping this desktop-only implementation open would duplicate upstream maintenance. Thanks.