feat: add goal view wallpaper flow

[Osamariyad2003]

Summary

• turn the former student view into a goal view flow
• switch goal setup from duration-based input to explicit start and end dates
• make the generated goal wallpaper match the simpler year-view visual style
• add a lightweight goal wallpaper preview link beneath the generated URL

Details

• update the shared goal input model and server-side parsing to support goalEndDate
• refresh the public page and dashboard goal form to use simpler textfields
• simplify the goal renderer so it feels consistent with the existing wallpaper modes
• add a dedicated GoalWallpaperPreview component and place it in the result area
• remove the extra goal-specific install action so the output flow stays minimal

Verification

• npx eslint components/GoalWallpaperPreview.tsx
• GET / returns 200 OK
• GET /api/wallpaper?...&viewMode=student&goalEndDate=2026-07-31 returns 200 OK as image/png