fix: give session action menu an opaque backdrop

[pi-claw-agent]

Summary

Session action menus no longer rely on the (often transparent) sideBarSectionHeader token: the menu now paints an editorWidget/editor-background fallback plus a near-opaque color-mix backdrop over the sidebar background, so long session lists no longer bleed through the open menu while a slight translucency is preserved.

Validation

• bun install --frozen-lockfile
• bun run check-types
• bun run lint
• bun run compile-tests
• bun esbuild.js
• bun esbuild.webview.js
• xvfb-run -a bun x vscode-test (200 passing)
• git diff --check

Closes #75

[pi-claw-agent]

Independent Agent review: approved

Review round 1/2 for 91df1b77746f.

The session action menu background no longer depends on the possibly-transparent sideBarSectionHeader token: it paints an opaque editorWidget/pi-bg fallback followed by a near-opaque (94%) color-mix backdrop over the sidebar background, so covered session rows no longer bleed through while a slight translucency is preserved. The change is confined to the .session-menu rule; elevation shadow and border are kept, and focused regression tests pin the new declarations.

Reviewer checks

• Inspected git diff main...91df1b7, the --pi-panel/--pi-bg token chain, and the menu rule within the sidebar webview styles.
• bun run compile-tests
• ./node_modules/.bin/mocha --ui tdd out/test/session-menu-backdrop.test.js (2 passing)
• ./node_modules/.bin/eslint src/test/session-menu-backdrop.test.ts
• bun run check-types
• git diff --check main...91df1b7

Generated by the independent sandboxed Reviewer Agent. The PR still requires human review and merge.