feat(m4): crop_fill setting + tape-line prototype (#99)

Add global crop_fill percent (default 90, 1-100) as a settings key with
FastAPI 4xx validation. Wires the value through to the inventory PNG
encode (non-retro: stored PNGs untouched). Adds a throwaway
TapePrototype.svelte behind ?tape-prototype for the #99 design sandbox.

Gates: check_requirements PASS, check_coverage PASS, settings tests 8/8.

frontend/package-lock.json

@@ -8,6 +8,7 @@
       "name": "slabuploader-frontend",
       "version": "0.1.0",
       "dependencies": {
+        "@neodrag/svelte": "2.3.3",
         "idb-keyval": "^6.3.0",
         "svelte": "^4.2.20"
       },
@@ -495,6 +496,15 @@
         "node": "^22.20 || ^24.12 || >=25"
       }
     },
+    "node_modules/@neodrag/svelte": {
+      "version": "2.3.3",
+      "resolved": "https://registry.npmjs.org/@neodrag/svelte/-/svelte-2.3.3.tgz",
+      "integrity": "sha512-avXzhrilsBsnMFljhVAQ7h+6hbSIrvRCJ61GCiGbGISkC1QOhjDCNvPZo2+7KVwiYrnUBx4NRH0kTIqrcxv9Lg==",
+      "license": "MIT",
+      "peerDependencies": {
+        "svelte": "^3.0.0 || ^4.0.0 || ^5.0.0"
+      }
+    },
     "node_modules/@playwright/test": {
       "version": "1.63.0",
       "resolved": "https://registry.npmjs.org/@playwright/test/-/test-1.63.0.tgz",