fix: anchor mention popover at composer top-leading

[datlechin]

Summary

Mention popover anchor was .point(.top) (top-center of composer), so the popover floated centered above the composer regardless of where the @ character was actually typed. Since the user types @ at the leftmost edge of the text input, the popover should hang over that edge instead of the center.

Change to .point(.topLeading). Combined with arrowEdge: .bottom, the popover now anchors at the top-left corner of the composer with its arrow tip at that point, hanging up-and-right from there.

This is the closest pure-SwiftUI approximation of caret-anchored autocomplete (Apple Mail address autofill, VS Code mention dropdown). True caret-precise anchoring requires NSTextView selection rect math, which would reintroduce the TextKit 1 dependency we dropped in #1076.

Test plan

• In the AI Chat composer, type @. The mention popover appears anchored to the top-left of the composer.
• Type more characters to filter; popover position stays stable.
• Press Esc to dismiss; popover hides cleanly.

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