iPadOS 26: update the main screen sidebar

[yexingsha]

Since iPadOS 26 is now much closer to a desktop OS, and relies more on the sidebar for navigation, we should bring the sidebar more in line with the desktop client:

• The biggest change is folding library navigation into the sidebar, just like the Collection Tree on desktop. Group libraries are put in the same section. We can disable dragging items into other libraries for now.
• Without the need for a Back button, the Settings button can be put in its place.
• Native apps with search in sidebars always show the search field on top and have it stay fixed on scroll. If we feel like that is one search field too many, then we can keep the current pattern with some minor fixes (larger padding, soft scroll edge effect, persisting cancel button to exit search mode).
• See iOS 26: update Collection component #1171 for changes to collection rows.
• In the tag selector, the search field needs to be fixed at the top and adopt the sidebar style (instead of layering liquid glass, which is discouraged), the overflow button should use the default color, and the entire top part should use a soft scroll edge effect.
• Right now, when the tag selector is fully collapsed, it can be impossible to bring it back up due to conflict with the Dock touch zone. An easy way to fix that is letting the collapsed tag selector still show the search field and button (see: Maps).

[michalrentka]

Just a little comment, when I first look at this, when I see the straight line split between tags and collections, it looks a bit out of place. I didn't spend much time in iOS 26 yet, so I don't know how it's "supposed to look" (on native apps for example), but maybe try rounded corners for tags too? Or even a shadow? It would pretty much look the same as sidebar in the main screen.

[yexingsha]

I have looked through most native apps, and none of them have this sort of split sidebar; sections all belong to the same list under section headers (e.g. Tags in Files, sections with horizontal scroll in Maps). A sub-container with rounded corners does look better:

I don't think a shadow is necessary here, but since the glass effect in Figma is a crude approximation, we will need to build this to see how it really looks like.