Show partial relation memberships in multiselection

[Shyam-123pandey]

Mergin.fix.mp4

Currently, when multiple features are selected, the relation panel only shows relations shared by all selected entities. This hides relations that belong to only some of the selected features.
This PR changes the logic to collect the union of parent relations instead of the intersection, so all relation memberships are shown in the panel.
I also updated the relation count so it correctly reflects the number of displayed relations.
Example
Before:
Way A → R1
Way B → R2

Multiselect → Relations (0)

After:
Way A → R1
Way B → R2

Multiselect → Relations (no of relations) showing R1 and R2
Tested locally using real data with roads belonging to different bus route relations.
And at the bottom add:
Fixes #9198

[Copilot]

Pull request overview

Updates the raw membership editor UI logic to compute parent relations across selected entities using a union of all parent relations instead of only the shared/intersected ones, affecting what relations appear and how they’re counted/displayed.

Changes:

• Replaced shared-parent (intersection) relation gathering with a new getAllParentRelations() implementation (union + dedupe by relation id).
• Updated the section label count and membership list generation to use the new parent relation gathering logic.

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[Shyam-123pandey]

Please review this PR @tyrasd .