fix(admin): DOM dropdown fallback so selects work in the desktop webview embed

[badbread]

The bug

The desktop client embeds the web admin console (services/api/src/admin.html) in a webview_windows webview that runs in visual-hosting / composited mode. That mode cannot display the OS-drawn popup of a native <select>, so clicking any dropdown in the embedded server-settings console does nothing. All ~53 <select> elements are affected. In a normal browser they work fine; only the desktop embed is broken.

The fix: an embed-only DOM-dropdown shim

Rather than rewrite 53 selects, this adds one small, reusable shim to admin.html:

• Activates only when embedded. It gates on the existing isEmbedded() helper (body.embedded, set by bootSSO from ?embedded=1 and/or #embed=1) that the rest of the file already uses. In a normal browser the shim is completely inert and native selects stay native.
• Event delegation, not per-select wiring. A single capture-phase mousedown/keydown interceptor on document covers selects added dynamically (camera editor, notifications pane, wizard) for free.
• Replaces only the popup. On opening a select in embed mode it preventDefault()s the dead native popup and renders a DOM overlay of that select's options, positioned under the control (flips above / clamps into the viewport). The native <select> stays in the DOM: its closed state paints fine and holds the value.
• Zero-change integration. Choosing an option marks the option selected and dispatches real bubbling input + change events, so every existing onchange= handler and .value read keeps working untouched.
• Closes on selection, outside click, Esc, scroll and resize. Keyboard open/nav/commit supported. Respects disabled selects and disabled options. Renders <optgroup> labels (none in the file today, but handled).
• Styled with the console's existing tokens (--surface2, --border2, --accent-soft, --accent, --shadow-lg).

Verification

• Extracted the inline <script> and ran node --check (passes).
• Built crumb-api on the dev box with the modified admin.html (green; include_str! embed intact).
• Exercised the shim in a browser via a harness using the exact CSS+JS: basic open, preventDefault of the native popup, option select firing change and updating .value, preselected-value highlight, disabled option not selectable, disabled select not opening, optgroup rendering, outside-click / Esc / scroll close, keyboard open+nav+commit, and confirmed it is fully inert (no preventDefault, no overlay) when body.embedded is absent.

How to verify in a plain browser

Load http://<host>:8080/admin?embedded=1#embed=1, then open any dropdown in Server Settings. It should open as a DOM list (not the native popup), and picking an option should update the setting (the underlying change handler fires and persists). Loading /admin without the embed flag keeps native selects unchanged.