perf: hot path in page load

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This quite literally removes one entire blocking redundant roundtrip from reaching DOM Load event (the hot path):

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Works even better with:

• https://github.com/frappe/frappe/pull/26752

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@barredterra I implemented this with yet another strategy, clarifying the contrived python implementation to prevent future developers from misunderstanding the JavaScript code.

The performance goal is now achieved through loading this (vastly) static parameter into the boot dictionary, essentially amounting to client side caching of this often-queried endpoint.

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@barredterra Could you revise your request for changes to this PR? Otherwise, it's ready and I am to own it and stand by for any potential consequences not covered by the test suite.