feat(app): add session panels

[oshtz]

Issue for this PR

Addresses part of #18287

Type of change

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

What does this PR do?

Adds an optional tiled panel mode for open session tabs in the desktop app's new UI.

• Reuses the existing shared composer and sends prompts to the focused panel.
• Keeps inactive panels readable but slightly dimmed.
• Supports horizontal and vertical panel resizing.
• Uses a 3-panel layout where the right panel spans the available height.
• Supports swapping panel positions by dragging from each panel header.
• Keeps transcript content constrained to each panel width.

How did you verify your code works?

• npx -y bun@1.3.14 test --preload ./happydom.ts ./src/pages/session/panel-layout.test.ts ./src/context/tabs-order.test.ts from packages/app
• npx -y bun@1.3.14 run typecheck from packages/app
• npx -y bun@1.3.14 run test from packages/app
• npx -y bun@1.3.14 run build from packages/app
• npx -y bun@1.3.14 run typecheck from packages/desktop
• npx -y bun@1.3.14 run build from packages/desktop

Screenshots / recordings

Session panels demo recording

The recording covers a clean project, three sessions, panel mode, horizontal and vertical resizing, dragging a panel header to swap positions, and sending a follow-up from the shared composer to the focused panel.

Checklist

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

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