fix(settings): allow text selection in settings panel

[quiet-node]

Summary

• Settings panel previously had user-select: none on the root .window and a drag handler whose interactive-tag opt-out missed text tags (<p>, <span>, <code>, headings). Click-and-drag on text moved the whole NSPanel instead of starting a selection, so users couldn't highlight or copy any text inside Settings.
• This PR keeps the chrome (top padding, tab bar) draggable while making body and banner text selectable, and teaches the drag handler to bail when the mousedown target is a text-bearing leaf.

What changed

• src/styles/settings.module.css: .body and .banner now declare user-select: text. The window-wide user-select: none on .window still applies to chrome.
• src/settings/SettingsWindow.tsx handleDragStart:
  • Bail when e.button !== 0 so right/middle-click pass through cleanly.
  • After the existing interactive-tag walk, bail when the target element directly contains a non-empty text node. Layout wrappers (<div>, <section>) without their own text still drag, so empty regions and gaps preserve the "click anywhere blank to drag" feel.
• Tests: two new cases in SettingsWindow.test.tsx cover the text-leaf no-drag path (banner <code>) and the non-primary-button no-drag path.

Why this design

• CSS-driven selectability matches the natural macOS expectation: text is selectable, chrome is not.
• Keeping the drag handler in JS (rather than data-tauri-drag-region) preserves the existing fallback for nested form children, which the attribute can't handle reliably at the root.
• "Direct text node" is a tighter signal than "any descendant has text" — it lets selectability and dragability coexist on the same parent without the window snatching the cursor mid-highlight.

Test plan

• bun run test:all:coverage — frontend + backend pass, coverage gate holds (100% lines/functions).
• bun run validate-build — lint, format, typecheck, build all clean.
• Manual: open Settings, drag empty area → window moves; click-drag across a label/value → selection starts; Cmd+C copies; Cmd+, and Cmd+W still work; tab buttons and form inputs still respond to clicks.