Adds support for `content_type=combined` in URL search parameters,
letting users force combined-mode searches via a bookmarkable link
rather than relying on the last-used preference from localStorage.
The override is applied only in Universal mode and only when combined
mode is actually available (universal enabled, `show_combined_selector`
on, neither content type blocked by policy). Otherwise, it's silently
ignored, consistent with how `content_type=ebook`/`audiobook` already
behave outside Universal.
Existing `content_type=ebook`/`audiobook` URLs now also force combined
mode off, so the URL is authoritative regardless of prior preference.
Also adds a per-user `FORCE_COMBINED_SEARCH` setting that locks combined
mode on whenever it's available.
URL `content_type=ebook`/`audiobook` overrides are also superseded by
force-combined for the same reason: the search bar wouldn't let users
switch back, so honoring the URL param would leave them in a state they
couldn't escape from.
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