fix(create): left-panel selections flicker and do not stay selected

[ShauryaaSharma]

Fixes #10910

Problem

On the /create page, selecting any option from the left-side configuration
panel (Base Color, Theme, Style, Font, Radius, etc.) would flicker and not
stay selected. The selection would briefly appear then reset, making the
customizer unusable.

Root Cause

Two compounding issues:

1. Non-shallow routing on every selection

useDesignSystemSearchParams defaulted to shallow: false, which triggered
a full Next.js server navigation on every picker click. This caused the page
to re-suspend mid-render, visually resetting the panel state while the URL
update propagated.

2. Racing setParams call in ThemePicker

ThemePicker had a useEffect that fired setParams({ theme: themes[0].name })
whenever currentTheme was undefined. After a baseColor change, there is a
brief render window where currentTheme appears undefined — causing a second
competing setParams call that raced against the first and produced the
back-and-forth oscillation.

Fix

search-params.ts — changed default to shallow: true in
useDesignSystemSearchParams. This matches how DesignSystemProvider already
calls it and prevents unnecessary server navigations on picker selections.

theme-picker.tsx — removed the redundant useEffect. Theme validation
and fallback is already handled authoritatively by normalizeDesignSystemParams
in search-params.ts, so the effect was never needed and only introduced a race condition.

Testing

Tested on http://localhost:4000/create?template=start across all browsers:

• Base Color, Theme, Style, Font, Heading Font, Radius, Icon Library, Menu Color
  all hold selection without flickering
• Changing Base Color correctly resets Theme to a valid value without oscillating
• Rapidly switching between options produces no flicker or state reset
• URL correctly updates to ?preset=CODE format and selection persists on reload

[vercel]

@ShauryaaSharma is attempting to deploy a commit to the shadcn-pro Team on Vercel.

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