Dock: hover-only title buttons, tilted pin, mirrored float grip

- Docked title bars show the pin and close only while the pointer is
  over the bar; the bar keeps a fixed height so nothing jumps.
- Auto-hide on is drawn as a 30° tilted pin (new pin_active.svg)
  instead of a coloured button background, on docked and floating bars.
- A floating panel's resize grip sits in the bottom corner away from its
  title bar. With the bar on the right it is bottom-left (new mirrored
  resize_left.svg) and resizing from there keeps the right edge fixed.
  The bar stays on its side while resizing.
- While a floating panel is dragged, only the drop preview shows it; the
  panel no longer stays drawn at its old place.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_013RyGiMXcsf9HTKfqDxaQGt

assets/icons/ui/pin_active.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="#000000" stroke-width="1.9" stroke-linecap="round" stroke-linejoin="round"><g transform="rotate(30 12 12)"><path d="M8 3 H16 L15 9 L18 12 V14 H6 V12 L9 9 Z"/><path d="M12 14 V21"/></g></svg>

assets/icons/ui/resize_left.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="#B4B6B9" stroke-width="1.8" stroke-linecap="round" stroke-linejoin="round"><path d="M3 9 L15 21 M3 15 L9 21 M3 21 L4 21"/></svg>