GlobalSearch: support tag-based predefined-value filters (tagFilters)

[abulte]

Problem

Some use cases require multiple user-facing select filters that all contribute values to the tag API param simultaneously. For example, the indicators page has theme, enjeu, and secteur filters — each a predefined-value select — that must each independently add a tag value to the API call without overriding each other or the hidden tag universe filter.

Currently this is impossible to express in GlobalSearchConfig. The #filters slot offers custom UI but slot components have no way to inject additional API params into GlobalSearch's query. The native tag filter is facet-based and doesn't support predefined values.

Ref: opendatateam/udata-front-kit#1153

Why tag-specific

tag is architecturally special in the data.gouv.fr API — it's the designated multi-value classification mechanism. Other filter params (organization, format, license…) are single-select and map 1:1 with their URL params. A generic urlParam → apiParam mapping system would be YAGNI.

Proposed config

{
  class: 'datasets',
  hiddenFilters: [{ key: 'tag', value: 'ecospheres-indicateurs' }],
  tagFilters: [
    { urlParam: 'theme', label: 'Thématique', defaultLabel: 'Toutes les thématiques', values: [
      { value: 'ecospheres-indicateurs-theme-mieux-consommer', label: 'Mieux consommer' },
      ...
    ]},
    { urlParam: 'enjeu', label: 'Enjeu', defaultLabel: 'Tous les enjeux', values: [
      { value: 'ecospheres-indicateurs-enjeu-adaptation-climat', label: 'Adaptation climat' },
      ...
    ]},
  ]
}

• Each tagFilter has its own URL param, isolated from others
• Selected values are pre-baked tag strings (prefix handling is the caller's responsibility)
• GlobalSearch renders the select UI, manages URL state, and merges all selected values with hiddenFilters and the native tag filter when building the API call