Integrate categories page with unified search controller

[Copilot]

Categories page links directly navigate to category-filtered results instead of routing through a unified search. This creates inconsistent UX and duplicated pagination/filtering logic.

Changes

Template updates

• categories.html: Redirect all category/subcategory links to /search/category?slug={slug} instead of /categories?category={slug}

Controller redirects

• CategoriesController: Add redirect from /categories?category={slug} → /search/category?slug={slug}
• New endpoint /categories/{slug} → /search/category?slug={slug} for direct category URLs
• Preserve pagination parameters across redirects
• Use UriComponentsBuilder for safe URL parameter encoding

Testing

• Add CategoriesControllerTest with coverage for redirects, pagination, hierarchical categories, and edge cases

Example

@GetMapping("/categories/{slug}")
public String categoryBySlug(@PathVariable("slug") String slug,
                             @RequestParam(value = "page", defaultValue = "0") int page) {
    UriComponentsBuilder builder = UriComponentsBuilder.fromPath("/search/category")
            .queryParam("slug", slug);
    if (page > 0) {
        builder.queryParam("page", page);
    }
    return "redirect:" + builder.build().toUriString();
}

Existing SearchController /search/category endpoint and shop-list.html pagination already handle category-filtered results correctly.

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