[FEATURE] Add "Mark" action to text selection menu for highlighting reading position

Problem

When reading a long response in Claude Desktop and getting interrupted (switching windows, taking a break), there is no way to mark or recover the reading position. Scrolling back to find where you left off in a lengthy output is a recurring friction point, especially in sessions with multiple long responses.

Proposed solution

When selecting text in a response, the floating action menu currently shows "Start a side chat" and "Reply". Add a third option — "Mark" (or "Highlight") — that applies a persistent highlight to the selected text.

Behaviors:

• Highlighted text gets a visible background tint (e.g. subtle yellow/amber)
• Multiple highlights per conversation are fine, but even a single-mark mode works
• A keyboard shortcut (e.g. Alt+J) to jump to the last marked position
• Clicking "Mark" on already-highlighted text clears it
• Alternatively (or additionally), expose this via the right-click context menu

This fits naturally into the existing selection interaction without adding new gestures or UI chrome.

Alternatives considered

• Message-level bookmarks (#67206, #32874): useful for different reasons, but don't solve intra-message "where was I reading" problem
• Scroll position memory: doesn't help when new content pushes the viewport
• External tools / browser extensions: fragile, out-of-ecosystem

Context

Any workflow involving reviewing long outputs — code reviews, research summaries, step-by-step guides — benefits from this. The selection menu is already the natural touch-point for interacting with specific text, so extending it with a highlight action feels low-friction to discover and use.