Split from #11146.

Problem

The Control UI has a command-palette shortcut, but common dashboard chat actions still require mouse interaction. The most obvious gap is the New messages button, which is click-wired only.

Current source evidence:

• ui/src/ui/app.ts has a document-level shortcut only for Cmd/Ctrl+K / Ctrl+K to toggle the command palette.
• ui/src/ui/views/chat.ts renders New messages with only the button click path to onScrollToBottom.
• Chat textarea key handling is local to composer behavior such as slash menu, search, and history; it is not a guarded dashboard dispatcher.

Scope

Add a guarded Control UI shortcut dispatcher for the smallest valuable set of dashboard actions:

• / focuses the chat composer when focus is not already inside editable content.
• N jumps to new messages / bottom when the new-message affordance is present.
• Esc dismisses transient dashboard UI such as command palette, search, menus, notices, or focus state where applicable.
• Keep existing Cmd/Ctrl+K command palette behavior.

Avoid binding Space as the primary jump shortcut for the first pass because it conflicts with native button/page behavior.

Acceptance criteria

• Shortcuts do not fire while focus is in an input, textarea, select, contenteditable region, dialog text field, or component-owned text editor.
• Existing composer-local arrow/history/slash-menu behavior remains unchanged.
• New-message jump reaches the same behavior as clicking New messages.
• Shortcut help/discoverability is available through the command palette or adjacent UI if there is an existing pattern for it.
• Focused UI tests cover guarded dispatch, no-op editable-focus cases, and the new-message path.

Validation expectation

Before merge, run focused UI tests for the touched shortcut/chat path and the repo's changed-file gate in Testbox for user-visible Control UI work.