fix: handle array-returning Vite plugins in mergeConfigDedup

[pffigueiredo]

Description

mergeConfigDedup assumed every entry in the plugins array is a single Plugin object with a .name property. However, Vite's PluginOption type allows plugins to be arrays (nested), falsy values, or promises. Plugins like @tailwindcss/vite return Plugin[] — a standard Vite convention called "plugin presets." This caused mergeConfigDedup to fail because it accessed .name on an array.

FIXES

• Flatten nested plugin arrays recursively before deduplication (handles @tailwindcss/vite and similar)
• Filter out falsy values (false, null, undefined) — standard Vite convention for conditional plugins
• Deduplication by .name works correctly for all plugin types

Test Plan

Added unit tests in packages/appkit/src/utils/tests/vite-config-merge.test.ts covering:

• Deduplication of single plugins by name
• Flattening array-returning plugins
• Deduplication across flattened array plugins and single plugins
• Deeply nested plugin arrays
• Filtering out falsy plugin entries
• Mixed falsy and valid plugins

Manual Testing

• Built and used the local version of appkit to make sure this fixed the issue

[pkosiec]

@MarioCadenas please take a look, as I'm not sure if this doesn't conflict with your changes in the plugins area?