Move spotlight beside thread title

[brsbl]

Summary

• Move the split Spotlight toggle from the pane-edge controls to the thread title cluster
• Keep Spotlight unfilled while preserving its icon and accessible pressed state
• Use one shared 4px header-action gap for the active title surface, Spotlight, overflow, and pane-edge icon controls
• Use one shared thread-header icon token for Spotlight, overflow, panel, maximize, and close
• Keep Full Screen, close, and panel controls grouped at the pane edge
• Preserve macOS title-bar click handling and add regression coverage for control ownership, painted-edge spacing, and icon geometry

Verification

• Focused ThreadDetailHeader, PaneMaximizeButton, header-token, and SplitThreadArea regression suites pass
• App typecheck passes
• Responsive visual QA passes at 390x844, 768x900, 1440x900, and 3440x1440
• Built-in controls measure 16x16 glyphs in 28x28 button boxes; split-capable sizes retain exact 4px gaps through six Spotlight toggles

BB-Thread-ID: thr_p2y4jr9a5q

AGENT GENERATED: by GPT-5.6