feat: Add Full-screen mode without title bar on Ubuntu (x64 and ARM64)

### Feature summary

Add support for running the app in a borderless full-screen mode that hides the standard system title bar / window frame.

### What problem are you trying to solve?

On Ubuntu, the native window title bar consumes vertical screen space, reducing the available viewing area and breaking full-screen immersion.

### Proposed solution

Provide a toggleable full-screen option (e.g., via `F11` or menu toggle) that removes window decorations and hides the title bar entirely on Ubuntu 26.04 across both `x86_64` (AMD64) and `aarch64` (ARM64) architectures.

### Workflow impact

Improves usability on smaller screens and provides a distraction-free environment for focused workflows.

### Installation context

Ubuntu 26.04 OS on both AMD64 and ARM64 system architectures.

### Additional context

No files or tests are named. Start by locating the Ubuntu desktop window configuration and any existing fullscreen, menu, or keyboard handling; done means a toggleable borderless fullscreen mode works on Ubuntu for both x86_64 and aarch64 without the title bar.

索引モデルが issue の本文から書いたものです。