Bug: default theme "feature_based" causes poster generation failure

## Summary

The map generator at https://maptoposter.penk.in/ fails when the default theme value is submitted.

## Error Message

```text
Error: Generation failed: Error: Theme "feature_based" not found.

Steps to Reproduce

1. Open https://maptoposter.penk.in/.
2. Leave the default theme selection unchanged.
3. Click Generate Poster.
4. Observe the generation failure.

Actual Behavior

The generator submits the theme value feature_based.

The backend rejects the request because feature_based is absent from the supported theme list.

Expected Behavior

The generator should submit a valid default theme and generate the poster successfully.

Available Themes Shown by the System

autumn
blueprint
contrast_zones
copper_patina
emerald
forest
gradient_roads
japanese_ink
midnight_blue
monochrome_blue
neon_cyberpunk
noir
ocean
pastel_dream
sunset
terracotta
warm_beige

Likely Cause

The frontend appears to initialize the theme state with feature_based.

The backend appears to validate themes against the supported theme list above.

This creates a mismatch between the frontend default value and the backend theme registry.

Suggested Fix

Set the frontend default theme to a valid supported key, such as ocean, blueprint, or contrast_zones.

Add backend fallback handling for empty or invalid theme values so the request uses a valid default theme.

User Workaround

Manually selecting a visible theme such as Ocean, Blueprint, or Contrast Zones before generation may avoid the error.