fix(providers): bulk-add API keys no longer overwrite existing keys

Bulk-add named auto-generated keys by paste-line index, blind to existing
connection names. The backend upserts apikey connections by exact name
(connectionsRepo), so a colliding generated name silently replaced an
existing key instead of inserting a new one.

Add a collision-aware planner (src/shared/utils/bulkAdd.js) that gap-fills
the smallest free "<base> <n>" against both existing connection names and
names assigned earlier in the same batch, so a generated name is never
reused and the backend always inserts. Applies to auto-named lines, custom
name|apiKey lines, and Cloudflare name|apiKey|accountId lines.

Wire the planner into AddApiKeyModal and pass existing connection names
from the provider detail page. Add unit tests covering gap-fill, custom
names, Cloudflare 3-part format, and robustness.

src/app/(dashboard)/dashboard/providers/[id]/page.js

@@ -1690,6 +1690,7 @@ export default function ProviderDetailPage() {
         website={providerInfo?.website}
         proxyPools={proxyPools}
         error={addConnectionError}
+        existingNames={connections.map((c) => c.name).filter(Boolean)}
         onSave={handleSaveApiKey}
         onBulkDone={fetchConnections}
         onClose={() => {