Add svelte-heros icon library

- Install svelte-heros for consistent Heroicons usage
- Replace inline SVGs for password toggle with Eye/EyeOff icons
- Fix USE_EMBEDDED mode to serve frontend from filesystem

frontend/bun.lock

@@ -14,6 +14,7 @@
         "daisyui": "^5.5.19",
         "postcss": "^8.5.8",
         "svelte": "^5.0.0",
+        "svelte-heros": "^8.0.1",
         "tailwindcss": "^4.2.2",
         "typescript": "^5.0.0",
         "vite": "^5.0.0",
@@ -289,6 +290,8 @@
 
     "svelte": ["svelte@5.55.0", "", { "dependencies": { "@jridgewell/remapping": "^2.3.4", "@jridgewell/sourcemap-codec": "^1.5.0", "@sveltejs/acorn-typescript": "^1.0.5", "@types/estree": "^1.0.5", "@types/trusted-types": "^2.0.7", "acorn": "^8.12.1", "aria-query": "5.3.1", "axobject-query": "^4.1.0", "clsx": "^2.1.1", "devalue": "^5.6.4", "esm-env": "^1.2.1", "esrap": "^2.2.2", "is-reference": "^3.0.3", "locate-character": "^3.0.0", "magic-string": "^0.30.11", "zimmerframe": "^1.1.2" } }, "sha512-SThllKq6TRMBwPtat7ASnm/9CDXnIhBR0NPGw0ujn2DVYx9rVwsPZxDaDQcYGdUz/3BYVsCzdq7pZarRQoGvtw=="],
 
+    "svelte-heros": ["svelte-heros@8.0.1", "", { "peerDependencies": { "svelte": "^5.0.0" } }, "sha512-67rDThxmdbjvJxKDqDUnkTsmyI5JtXcS0S0Ryg/EmTlHscFyJNgJyEARX5/xt3ONsZA0i6+dWnq+EX5jH9Wq1Q=="],
+
     "tailwindcss": ["tailwindcss@4.2.2", "", {}, "sha512-KWBIxs1Xb6NoLdMVqhbhgwZf2PGBpPEiwOqgI4pFIYbNTfBXiKYyWoTsXgBQ9WFg/OlhnvHaY+AEpW7wSmFo2Q=="],
 
     "tapable": ["tapable@2.3.2", "", {}, "sha512-1MOpMXuhGzGL5TTCZFItxCc0AARf1EZFQkGqMm7ERKj8+Hgr5oLvJOVFcC+lRmR8hCe2S3jC4T5D7Vg/d7/fhA=="],