fix(ui): make Feast UI Vite-compatible by removing browser-only process/env and EUI dynamic icon breakage - #6724

Problem

Importing @feast-dev/feast-ui in a Vite app fails at runtime, blocking the documented module-integration path.

Observed failures included:

• ReferenceError: process is not defined
• Failed to fetch dynamically imported module: .../assets/arrow_down?import

Root cause

Two Vite-incompatible runtime paths were involved:

• Direct process.env reads in browser-executed UI code (FeastUI and CurlGeneratorTab).
• EUI icon import behavior that triggers unresolved arrow_down dynamic module loading in this packaging context.

Fix

• Added a safe env helper (getProcessEnv) and replaced direct process.env access in:
  • ui/src/FeastUI.tsx
  • ui/src/pages/feature-views/CurlGeneratorTab.tsx
• Replaced EuiSelect in ProjectSelector with a native <select> to avoid the dynamic icon import path that causes arrow_down resolution failures.
• Replaced the user-menu chevron icon usage in Layout with a local inline SVG glyph.
• Added/updated focused tests:
  • ui/src/utils/environment.test.ts
  • ui/src/components/ProjectSelector.test.tsx

Test plan

• cd ui && npx jest src/utils/environment.test.ts src/components/ProjectSelector.test.tsx src/FeastUISansProviders.test.tsx --runInBand
• cd ui && npm run build:lib

Risk / rollback

Risk is low-to-medium and limited to UI behavior:

• Minor styling/accessibility behavior differences from EuiSelect -> native <select>.
• User-menu chevron changed to inline SVG.

Rollback is straightforward by reverting this PR commit.