feat: move desktop settings into dedicated window

[ksamaschke]

Summary

• move theme controls and desktop preferences out of the main shell into a dedicated Settings window
• add a typed desktop bridge path for opening/focusing the reusable Settings BrowserWindow
• extract shared appearance/window-role helpers and remove obsolete main-window Settings ownership paths

Changes

• removed the main-window Settings tab and inline appearance popover from InstallerDashboard
• added windowRole-based renderer composition plus a dedicated Settings appearance component
• wired openSettingsWindow() through bridge, preload, control-plane client, and Electron main window management
• added focused Settings-ownership tests plus adjacent desktop bridge/client coverage

Test Plan

• npm run build:quick
• node --test dist/tests/installer/desktop-settings-window-red-phase.test.js
• node --test dist/tests/installer/control-plane-client.test.js dist/tests/installer/desktop-bridge-red-phase.test.js dist/tests/installer/desktop-preview-startup.test.js dist/tests/installer/desktop-shell-ux.test.js
• node --test dist/tests/installer/desktop-dashboard-build.test.js

Breaking Changes

• None

[ksamaschke]

ICA-REVIEW
ICA-REVIEW-RECEIPT
Reviewer-Stage: 3 (temp checkout)
Reviewer-Agent: reviewer (subagent)
PR: #252
Base: dev
Head-SHA: d848a15
Date-UTC: 2026-03-15T06:02:13Z

Findings: 0
NO FINDINGS

Checks/Tests:

• npm test (PASS)
• gh pr checks 252 (PASS)

Notes:

• Fresh temp checkout review completed against the PR head SHA.
• Dedicated Settings ownership is isolated to the Settings window; the main window now launches it instead of owning theme/preferences UI.

Result: PASS

ICA-SECURITY-REVIEW
ICA-SECURITY-REVIEW-RECEIPT
Security-Reviewer-Stage: post-pr (temp checkout)
Security-Reviewer-Agent: security reviewer (subagent)
Security-Reviewer-Execution: dedicated-security-subagent
Security-Reviewer-Executor: github:ksamaschke
Security-Reviewer-Run-ID: sec-20260315T060213Z-d848a1550690
PR: #252
Base: dev
Head-SHA: d848a15
Date-UTC: 2026-03-15T06:02:13Z

Findings: 0
NO FINDINGS

Result: PASS