- Device model: add custom_chrome_url nullable column
- API: GET /api/wmt/config/<mac> returns device custom_chrome_url when set,
falls back to WMTGlobalConfig.chrome_url for all other devices
- Web: device_edit POST saves/clears custom_chrome_url from form
- Template: wmt/device_form.html adds Chrome URL override section with
clear status indicator (using global vs custom URL)
- DB: ALTER TABLE devices ADD COLUMN custom_chrome_url VARCHAR(500) applied
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