Add selection-aware actions to the text-selection context menu (Copy selection, Search Web, Go To URL)

Summary

When you highlight text in a message and right-click, the context menu only offers message-level actions:

• Copy message
• Copy as Markdown
• Attach selection as context
• Pin as chapter

There is no action that operates on the highlighted selection itself. This request is to add the selection-aware actions a user expects from a normal text-selection menu.

Requested menu items (when a text selection exists)

1. Copy selection — copy exactly the highlighted text (plain text, no styling).

• Today the menu has no plain "Copy"; Copy message grabs the whole message instead. ⌘C/Ctrl+C works as a workaround but the right-click path is missing.
• Closely related to / overlaps the existing copy-selection issue #50386 (and #63228, closed as a dup of it). This request is the broader "selection-aware menu" version; the plain-copy piece can be folded into #50386 if preferred.

1. Search Web for "<selection>" — open a web search for the highlighted text (same idea as the OS/browser "Search Web for …" item).
2. Go To <url> — when the selection is (or contains) a URL, offer to open it. Useful for links rendered in plain text / code blocks that aren't clickable.

Why

These are the standard right-click actions in basically every text surface (browsers, native macOS text views, editors). Their absence makes the selection menu feel incomplete — the most common thing you'd want to do with a highlighted snippet (copy it, look it up, follow it) isn't there.

Suggested behavior

• Only show these items when there's an active text selection (and gate "Go To" on the selection resolving to a URL).
• Keep the existing message-level items; these are additive.
• "Copy selection" should copy plain text (this also sidesteps the dark-background rich-paste problem noted in #63228).

Environment

• Claude Code desktop app, macOS (Darwin 25.x)