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/tools/color-adjustments.ts

@@ -1,7 +1,6 @@
 import {
   brightness as adjustBrightness,
   contrast as adjustContrast,
-  saturation as adjustSaturation,
   sharpen as adjustSharpen,
   colorChannels,
   grayscale,

apps/api/src/routes/tools/content-aware-resize.ts

@@ -65,7 +65,7 @@ export function registerContentAwareResize(app: FastifyInstance) {
         try {
           fileBuffer = await decodeHeic(fileBuffer);
           const ext = filename.match(/\.[^.]+$/)?.[0];
-          if (ext) filename = filename.slice(0, -ext.length) + ".png";
+          if (ext) filename = `${filename.slice(0, -ext.length)}.png`;
         } catch (err) {
           return reply.status(422).send({
             error: "Failed to decode HEIC/HEIF file",

apps/api/src/routes/tools/image-enhancement.ts

@@ -4,7 +4,6 @@ import sharp from "sharp";
 import { z } from "zod";
 import { autoOrient } from "../../lib/auto-orient.js";
 import { validateImageBuffer } from "../../lib/file-validation.js";
-import { sanitizeFilename } from "../../lib/filename.js";
 import { decodeHeic } from "../../lib/heic-converter.js";
 import { resolveOutputFormat } from "../../lib/output-format.js";
 import { createToolRoute } from "../tool-factory.js";

apps/api/src/routes/tools/passport-photo.ts

@@ -160,7 +160,7 @@ export function registerPassportPhoto(app: FastifyInstance) {
         if (validation.format === "heif") {
           fileBuffer = await decodeHeic(fileBuffer);
           const ext = filename.match(/\.[^.]+$/)?.[0];
-          if (ext) filename = filename.slice(0, -ext.length) + ".png";
+          if (ext) filename = `${filename.slice(0, -ext.length)}.png`;
         }
 
         // Auto-orient to fix EXIF rotation

apps/api/src/routes/tools/remove-background.ts

@@ -83,7 +83,7 @@ export function registerRemoveBackground(app: FastifyInstance) {
         if (validation.format === "heif") {
           fileBuffer = await decodeHeic(fileBuffer);
           const ext = filename.match(/\.[^.]+$/)?.[0];
-          if (ext) filename = filename.slice(0, -ext.length) + ".png";
+          if (ext) filename = `${filename.slice(0, -ext.length)}.png`;
         }
 
         // Auto-orient to fix EXIF rotation

apps/api/src/routes/tools/stitch.ts

@@ -205,8 +205,9 @@ export function registerStitch(app: FastifyInstance) {
 
       if (settings.cornerRadius > 0) {
         const meta = await sharp(result).metadata();
-        const w = meta.width!;
-        const h = meta.height!;
+        if (!meta.width || !meta.height) throw new Error("Cannot read image dimensions");
+        const w = meta.width;
+        const h = meta.height;
         const r = Math.min(settings.cornerRadius, Math.floor(Math.min(w, h) / 2));
 
         const mask = Buffer.from(