fix: Hide preinstalled example Snap

[FrederikBolding]

The previous versions of the preinstalled example Snap had omitted hidden, this results in it actually being hidden from the UI, which some E2Es assert against. This PR restores that behaviour and makes it explicit.

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Note

Low Risk
Single boolean in an example snap config; no production snap controller or security behavior changes.

Overview
Sets preinstalled.hidden to true in the preinstalled example’s snap.config.ts so the snap stays out of the normal UI, matching how older builds behaved when the flag was omitted and what E2E tests expect.

No runtime logic changes—only explicit config for the example package.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.59%. Comparing base (d4cf201) to head (b87e128).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4068   +/-   ##
=======================================
  Coverage   98.59%   98.59%           
=======================================
  Files         428      428           
  Lines       12484    12484           
  Branches     1975     1975           
=======================================
  Hits        12309    12309           
  Misses        175      175           

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