fix(lint): resolve all biome warnings across API, web, and image-engine

API:
- Replace string concatenation with template literals (batch, pipeline,
  tool-factory, content-aware-resize, passport-photo, remove-background)
- Remove unused imports (teams, color-adjustments, image-enhancement)
- Replace non-null assertions with guard clauses in bg-effects and stitch
- Use optional chaining in docs route

Image-engine:
- Remove unused OutputFormat imports (compress, convert)
- Use local variables instead of reassigning parameters (optimize-for-web, sharpen)

Web:
- Fix useExhaustiveDependencies: remove genuinely redundant deps, add
  biome-ignore comments for intentional patterns (src prop, cleanup fns)
- Replace non-null assertions with null-safe alternatives
- Add accessible titles to inline SVGs (color-settings, image-enhancement-settings)
- Fix noLabelWithoutControl: associate labels via htmlFor/id or use <span>
  (image-to-base64-settings, edit-metadata-settings, qr-generate-settings)
- Replace div[role="button"] with <button> (pdf-to-image-settings)
- Use stable keys instead of array indices (collage-preview, collage-settings)
- Fix noStaticElementInteractions in collage-preview (role="none")
- Remove unused function parameters and imports

apps/api/src/routes/batch.ts

@@ -153,7 +153,7 @@ export async function registerBatchRoutes(app: FastifyInstance): Promise<void> {
                 processBuffer = await decodeHeic(processBuffer);
                 // Update extension to match decoded format (HEIC/HEIF → PNG)
                 const ext = processFilename.match(/\.[^.]+$/)?.[0];
-                if (ext) processFilename = processFilename.slice(0, -ext.length) + ".png";
+                if (ext) processFilename = `${processFilename.slice(0, -ext.length)}.png`;
               }
               if (!skipPreprocess) {
                 processBuffer = await autoOrient(processBuffer);