fix: mitigate Linux X11 blank WebKit renders

[Dimillian]

Motivation

• Work around sporadic blank main-window renders observed on X11/WebKitGTK (app becomes visually blank when switching focus) by disabling WebKit compositing at process start on Linux.

Description

• Set WEBKIT_DISABLE_COMPOSITING_MODE=1 when unset during startup on Linux in src-tauri/src/lib.rs to avoid compositing-related blanking; preserves the existing __NV_PRIME_RENDER_OFFLOAD workaround.

Testing

• No automated tests were run because this is an environment-only startup change.

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Codex Task

[Dimillian]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. You're on a roll.

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