refactor: rebuild chat composer on SwiftUI TextField

[datlechin]

Summary

PR B of the chat panel redesign tracked in docs/refactor/ai-chat-redesign.md. Drops the 292-LOC ChatComposerTextView (NSViewRepresentable around NSTextView + NSScrollView + manual NSLayoutManager caret math + NSPopover + NSHostingController) and rebuilds the composer on pure SwiftUI primitives.

What changed

• ChatComposerView (new, 189 LOC): TextField(text:, axis: .vertical) + .lineLimit(1...5) + .onSubmit + .onKeyPress(.upArrow / .downArrow / .tab / .escape) + native SwiftUI .popover. Pill-shaped Capsule background. Apple Intelligence focus glow on @FocusState change.
• Apple Intelligence shimmer (~70 LOC, inline private types): 4 layered angular-gradient strokes (lineWidths 1.5/3/5/7, blurs 0/3/8/12, opacities 1.0/0.7/0.45/0.25) with stops randomized every 400ms and per-layer animation durations of 0.5/0.6/0.8/1.0s. Color palette is Apple's documented Intelligence pastel set (BC82F3 purple, F5B9EA pink, 8D9FFF blue, FF6778 red, FFBA71 orange, C686FF lavender). Respects accessibilityReduceMotion.
• Chat actions move to inspector header: conversation history (clock) and new-conversation (square.and.pencil) move from the composer footer to the inspector header bar, trailing the Details / AI Chat segmented picker. Composer footer is now a single row of 5 actions: @, /, mode, model, send.
• Empty state uses the project's EmptyStateView (which wraps macOS 14 ContentUnavailableView) for consistency with every other empty state in the app.
• Drops ChatComposerTextView.swift (292 LOC), the NSTextView subclass, the NSScrollView container, the manual intrinsicContentSize math, and the deprecated TextKit 1 NSLayoutManager.glyphIndexForCharacter / boundingRect(forGlyphRange:) caret math.

Mention popover

The old composer anchored the mention popover at the caret using NSLayoutManager glyph rects. The new composer anchors it at the bottom of the composer (.popover(attachmentAnchor: .point(.bottom), arrowEdge: .top)). This matches Slack, Discord, Apple Mail address-autofill, and removes the only TextKit 1 dependency. MentionDetector, MentionPopoverState, and MentionSuggestionListView are unchanged.

Net code

• 292 LOC deleted (ChatComposerTextView.swift)
• 189 LOC added (ChatComposerView.swift, including the inline shimmer)
• ~80 LOC moved (history + new-conversation: composer footer → inspector header)
• 0 lines of NSViewRepresentable, 0 lines of TextKit 1, 0 lines of manual NSPopover

Test plan

• Build the app, open a connection.
• Switch the inspector to AI Chat. Confirm the inspector header shows Details | AI Chat on the left and clock | square.and.pencil on the right.
• Click the composer. Confirm the pill border picks up an animated multi-color glow that morphs (not a static rainbow, not a mechanical rotation).
• System Settings → Accessibility → Display → Reduce Motion → on. Click the composer. Confirm the glow is static (no animation).
• Type 5+ lines of text. Confirm the composer grows up to 5 lines then scrolls.
• Press Return. Confirm the message sends.
• Press Shift+Return. Confirm a newline is inserted.
• Type @. Confirm the mention popover opens anchored to the bottom of the composer.
• Up/Down arrows navigate; Return or Tab inserts; Escape dismisses.
• Click the clock icon in the inspector header. Confirm the history menu appears with recent conversations and a "Clear Recents" entry.
• Click the new-conversation icon. Confirm a fresh conversation starts.

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