Header scope filtering & bulk scope editing

[mkappelhof]

Adds URL-scope awareness to the header list, plus a guided flow for scoping multiple headers at once.

New Features

• URL filter bar — Filter the header list by URL scope to quickly find scoped headers
• Global tab wizard — Batch-target multiple headers to a specific URL via a guided flow
• Drag & drop URL targeting — Reassign a header's URL target by dragging it to a new scope
• i18n groundwork — All UI copy is now externalized to src/i18n (English only for now, contributions welcome!)

Improvements

• Consistently match scoped headers against domains, subdomains, paths, and wildcards
• Group headers on the URL-specific tab by their exact scoped URL
• Reuse previously used URLs via the new Select component when targeting a header
• Focus the header key input automatically after creating a new header
• Clearer empty states for the header list
• Prominent alert on the Global tab clarifying that those headers apply everywhere
• New Toast component — saving header info now gives instant feedback