feat: #3633799 Offer Generate with AI inside each media type tab, as a switch next to Upload (!30) · Merge requests · project / ai_media

Depends on !29 (merged) (#3633789), which this branch includes; review that one first. After !29 (merged) merges this rebases to one commit.

What changed

"Where should the AI tabs appear?" (tab_position) gets a new default, inside: the Media Library tab list is core's (Image, SVG Image, Video, …), and each type with a generator starts with a switch, Upload | ✦ Generate with AI (with the "New" badge until Generate has been seen once).
Each mode has its own URL (ai_media=1); in the Media Library the switch swaps the content over AJAX like core's tabs and puts focus back on the chosen mode.
The same switch tops the Add media page (/media/add/{type}): Upload shows core's form only, Generate shows the generator in place of the upload widget. Generate media and the type chooser open Generate mode.
Standalone tabs stay: "As a tab of their own, right after…" (after_type) and "…after all the other tabs" (end).
Styled with the admin theme's variables (Default Admin's --admin-*, Gin's --gin-*), labels never wrap, focus ring, forced colors. The design follows a UX spec for Gin and Default Admin.
No update hook: pre-release, the default is in config/install.

Kernel: MediaLibraryAiTabsTest::testInside (tabs, switch, modes, no switch without generator or permission). Full suite: 86 tests pass; phpcs and phpstan clean.
Manually in Gin (Varbase 11): Media Library Image, SVG Image and Video; Upload ↔️ Generate; Add media page both modes; a real SVG generated in Generate mode; Generate media page and chooser links.