fix(ui): plugin picker — show extra groups, search icon, labels

[BatLeDev]

Registry groups outside the configured pluginCategories were silently dropped from the picker, and the search icon wasn't rendering.

What changed

• new.vue: orderedGroups now appends any registry group not listed in pluginCategories (alphabetically, just before the trailing "Other" bucket) so plugins are never hidden because their group is unconfigured. Fixes the search field's magnify icon (string "mdi-magnify" → bound :prepend-inner-icon="mdiMagnify", which renders with the auto-imported icon set). Adds a title tooltip on artefact names, minor spacing (mt-2, my-4), and relabels strings "processing" → "plugin" (fr+en).
• schema.json: documents pluginCategories as display-order-only, noting that unconfigured groups are appended after the configured ones.

Why
A plugin whose registry group wasn't in pluginCategories disappeared from the picker entirely; the search icon also never showed because the bare mdi-magnify string isn't in the SVG icon set.

Regression risks

• Group ordering changes for any registry exposing groups outside pluginCategories (they now appear instead of being hidden); configured groups keep their leading order.
• User-facing wording changed ("plugin" vs "processing") in fr/en.