fix: make screenshot capture safe and bounded

[okhsunrog]

What changed

• implement native Linux viewport screenshots with WebKitGTK's visible-region snapshot API
• bound the html2canvas fallback to the visible viewport instead of allocating a full-document canvas
• require explicit allowScreenCapture: true before invoking the interactive Screen Capture API fallback
• add unit coverage and an E2E stress case with 10,000 offscreen DOM rows

Why

On Linux, native capture was deliberately disabled by an outdated glib-version constraint. The fallback rendered the entire document.documentElement, so long pages could exceed the bridge's JavaScript timeout. After that failure the server automatically called getDisplayMedia(), unexpectedly opening a KDE screen-sharing permission prompt.

Tauri 2.9 and WebKitGTK now use the same glib 0.18 dependency line, so native visible-region capture can be enabled without duplicate GTK stacks.

Validation

• npm run build
• npm test (152 passed, 1 platform skip)
• npm run eslint
• cargo fmt --check for plugin and test app
• cargo clippy --manifest-path packages/tauri-plugin-mcp-bridge/Cargo.toml --all-targets -- -D warnings
• manual Linux stress run: 10,091 DOM elements, 321,044 px document height, native PNG viewport captured in 89 ms