Overhaul inline assistant (#12846)

This pull request introduces a new diff mechanism that helps users
understand exactly which lines were changed by the LLM.

Release Notes:

- N/A

assets/icons/context.svg

@@ -0,0 +1,6 @@
+<svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
+<path d="M12.6667 2H3.33333C2.59695 2 2 2.59695 2 3.33333V12.6667C2 13.403 2.59695 14 3.33333 14H12.6667C13.403 14 14 13.403 14 12.6667V3.33333C14 2.59695 13.403 2 12.6667 2Z" stroke="#888888" stroke-width="1.25" stroke-linecap="round" stroke-linejoin="round"/>
+<path d="M9 5H5" stroke="#888888" stroke-width="1.25" stroke-linecap="round" stroke-linejoin="round"/>
+<path d="M10.5 8H5" stroke="#888888" stroke-width="1.25" stroke-linecap="round" stroke-linejoin="round"/>
+<path d="M9 10.9502H5" stroke="#888888" stroke-width="1.25" stroke-linecap="round" stroke-linejoin="round"/>
+</svg>

assets/icons/rotate_cw.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-rotate-cw"><path d="M21 12a9 9 0 1 1-9-9c2.52 0 4.93 1 6.74 2.74L21 8"/><path d="M21 3v5h-5"/></svg>

assets/icons/stop.svg

@@ -0,0 +1,3 @@
+<svg width="12" height="12" viewBox="0 0 12 12" fill="none" xmlns="http://www.w3.org/2000/svg">
+<path d="M9.88889 1H2.11111C1.49746 1 1 1.49746 1 2.11111V9.88889C1 10.5025 1.49746 11 2.11111 11H9.88889C10.5025 11 11 10.5025 11 9.88889V2.11111C11 1.49746 10.5025 1 9.88889 1Z" stroke="#C56757" stroke-width="1.25" stroke-linecap="round" stroke-linejoin="round"/>
+</svg>