Purpose

The PC builder loaded saved builds from localStorage without validating them, treated missing component prices as zero without telling the user, and showed several unnecessary "Unknown" compatibility messages for an empty build. This PR makes the builder more reliable and clearer in those cases.

Changes

• Validate each saved part before loading it. A part is kept only if its category matches the key it is saved under, it has a string id and name, its price is null or a non-negative number, and details is an object. Invalid parts are dropped and valid ones are kept. Broken JSON and values that are not an object still give an empty build, without crashing.
• Show "Incomplete total: N selected components have no price." under the build total when any selected component has no price. Slots for such components now say "Price unavailable" instead of a bare "€".
• Show a single message, "Add components to your build to check compatibility.", when no components are selected, instead of eight "Unknown" lines. Once any component is selected the normal compatibility panel, including warnings and incompatibilities, is shown as before.
• Update BuildProvider.test.tsx for the full part shape and the new validation.

Related issues

Closes #99

Verification

(frontend):

• npm test: all 89 tests pass.

Manual testing (steps to run in the browser):

1. With an empty build, the compatibility panel shows only the single message. With parts selected, the groups and any incompatibilities show as before.
2. Set one saved part's price_eur to null in localStorage and reload: the summary shows the incomplete-total note and the slot shows "Price unavailable".

Screenshots

Checklist

• I have reviewed my own changes.
• The changes meet the linked issue's acceptance criteria, where applicable.
• I have verified the changes and recorded the results above.
• I have added or updated tests where needed.
• I have updated documentation where needed.
• I have included screenshots for visual changes, or marked them not applicable.