test: comprehensive test coverage expansion (+965 tests)

Add 42 new test files covering all untested tool routes, image engine
internals, AI sidecar bridge, Zustand stores, and cross-format
compatibility. Expand e2e-docker suite with 7 spec files covering all
48 tools against a real Docker container.

Unit tests:
- Image engine: format detection, MIME mapping, metadata parsing, pipeline
- AI bridge: sidecar lifecycle, all 11 tool functions (mocked)
- Web stores: 14 Zustand stores (collage, settings, features, analytics, etc.)
- API helpers: format decoders, page range, file validation

Integration tests:
- 25 tool routes that had zero dedicated tests
- Cross-format matrix: 17 input formats x 3 tools
- Edge cases: zero-byte files, corrupted headers, path traversal, XSS, SQL injection
- Concurrent request handling and pipeline edge cases

E2E-Docker (Playwright against real container):
- 7 spec files: essential, adjustment, conversion, creative, utility, AI, pipeline
- Custom buildMultipart helper for multi-file tool uploads
- AI tools gracefully skip when sidecar not installed

Fixtures:
- Organized test media: formats/ (18 formats) + content/ (17 content types)
- Reduced from 3.1 GB unorganized samples to 33 MB structured fixtures

Bug fix:
- color-adjustments: gamma exposure used invalid single-param gamma() for
  positive values; fixed to use two-param gamma(gammaIn, gammaOut) form

tests/fixtures/content/svg-logo.svg

@@ -0,0 +1,38 @@
+<svg width="400" height="400" viewBox="0 0 400 400" fill="none" xmlns="http://www.w3.org/2000/svg">
+  <title>SVG Drop Shadow Filter Effects - ConvertICO</title>
+  <desc>Sample SVG demonstrating drop shadow and glow filter effects. Free download at ConvertICO.com</desc>
+  
+  <defs>
+    <linearGradient id="logoGradient" x1="35" y1="250" x2="465" y2="250" gradientUnits="userSpaceOnUse">
+      <stop stop-color="#EF4362"/>
+      <stop offset="0.5625" stop-color="#F04E63"/>
+      <stop offset="1" stop-color="#F16667"/>
+    </linearGradient>
+    
+    <filter id="glow" x="-50%" y="-50%" width="200%" height="200%">
+      <feGaussianBlur in="SourceGraphic" stdDeviation="6" result="blur"/>
+      <feMerge>
+        <feMergeNode in="blur"/>
+        <feMergeNode in="blur"/>
+        <feMergeNode in="SourceGraphic"/>
+      </feMerge>
+    </filter>
+  </defs>
+  
+  <rect width="400" height="400" fill="#1a252f"/>
+  <circle cx="50" cy="50" r="80" fill="#3399cc" opacity="0.1"/>
+  <circle cx="350" cy="350" r="100" fill="#339966" opacity="0.1"/>
+  
+  <!-- ConvertICO Logo with glow (exact paths) -->
+  <g transform="translate(80, 65) scale(0.48)" filter="url(#glow)">
+    <path d="M250 465C368.741 465 465 368.741 465 250C465 131.259 368.741 35 250 35C131.259 35 35 131.259 35 250C35 368.741 131.259 465 250 465Z" fill="url(#logoGradient)"/>
+    <path d="M381.7 222.9C394.9 222.9 405.6 212.2 405.6 199C405.6 185.8 394.9 175.1 381.7 175.1C368.5 175.1 357.8 185.8 357.8 199C357.8 212.2 368.5 222.9 381.7 222.9Z" fill="white"/>
+    <path d="M365.295 294.67C379.597 286.027 359.018 260.022 373.163 248.835C387.308 237.647 402.765 242.338 408.593 251.058C413.021 262.52 412.818 272.435 404.875 278.379C396.932 284.323 411.106 292.461 396.366 308.906C393.517 315.686 395.157 313.774 395.356 324.998C395.555 336.222 379.73 360.586 354.619 346.39C329.509 332.193 350.993 303.313 365.295 294.67Z" fill="white"/>
+    <path d="M359 154.2C350.1 164 335.1 165.2 324.8 156.9C322.2 154.8 319.4 152.8 316.6 150.9C274.1 121.5 212.7 124.7 173.5 158.4C164.8 165.7 157.2 174.1 150.9 183.5C121.5 226 124.7 287.4 158.4 326.6C165.7 335.3 174.1 342.9 183.5 349.2C221.1 375.2 273.5 375.7 312.2 352.2C321.5 346.5 333.4 347.8 341.5 355.1C344.9 358.1 348.3 361.3 351.6 364.2C357.4 369.5 357.1 378.7 350.9 383.5C239.4 469.7 69 379.6 83.5 234.7C93.8 96.1 269.1 33.7 365.5 128.8C370.3 133.6 370.6 141.3 366.1 146.3L359 154.2Z" fill="white"/>
+    <ellipse cx="389.5" cy="263" rx="22.5" ry="23" fill="white"/>
+  </g>
+  
+  <text x="200" y="320" font-family="Arial, sans-serif" font-size="28" font-weight="bold" text-anchor="middle" fill="#ffffff">CONVERTICO</text>
+  <text x="200" y="348" font-family="Arial, sans-serif" font-size="14" text-anchor="middle" fill="#666666">Filter Effects Sample</text>
+  <text x="200" y="380" font-family="Arial, sans-serif" font-size="11" text-anchor="middle" fill="#3399cc">convertico.com/samples/svg</text>
+</svg>