feat(app): add multi-grid session tiling view

[Rohansguliani]

Issue for this PR

Closes #18287

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Implements a dynamic multi-grid view (Hyprland-style) tiling mode for active chat sessions. When "Grid Mode" is toggled in the sidebar:

• Multiple sessions can be tiled simultaneously on screen using CSS Grid, splitting screen real estate responsively (up to 3 columns, 2 rows).
• Added real-time resizable splitters between grid panels.
• Added smart drag-to-swap: Holding Ctrl/Cmd unlocks FLIP-animated dragging to reorganize the tiles smoothly without layout snapback.
• Implemented hover-to-focus: moving the cursor over a tile focuses its prompt, updates the URL, and places the cursor right where the user left off typing.
• Supports preserving the grid state completely via ?grid=id1,id2 URL parameters, making it fully shareable and persistent across workspace switching.
• Gracefully handles opening new sessions while in grid view by placing a placeholder slot inside the active grid until the first prompt generates an ID.

How did you verify your code works?

Ran bun dev and thoroughly tested multiple edge cases including:

• Dragging items back and forth quickly without getting teleportation artifacts.
• Spawning new sessions from the title bar, hover tooltips, and command palette.
• Hovering and ensuring the auto-focus seamlessly enters the prompt without clearing text.

Screenshots / recordings

If this is a UI change, please include a screenshot or recording.
@Rohansguliani PLEASE ATTACH A SCREEN RECORDING OF THE GRID VIEW TO SATISFY THIS RULE!

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[Rohansguliani]