fix: keyboard-navigate command suggestions without swallowing send

Summary

Fixes #233. The suggestion list was rebuilt click-only: mentions got arrow keys, commands did not. Up and down move a highlight, tab inserts the highlighted command, escape hides the panel until the text changes.

Return still sends a finished command (/help) and a command that already has arguments (/msg alice). It only completes an incomplete prefix (/he becomes /help ), so opening the panel does not eat the key that runs the command.

Test plan

• CommandSuggestionNavigation checks (prefix vs finished alias vs arguments, arrow wrap) compiled and ran locally under Swift 5 and Swift 6.
• On macOS, type /h, arrow between /help and /hug, tab to insert, and hit return on /help itself to send.
• Mention completion (arrows, tab, return) still completes the nickname instead of sending.
• Could not run xcodebuild here (no Xcode). CI covers the app build and the new test file.

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