feat(frontend): add document history panel with diff and compare mode

Adds a collapsible history section to the document detail view, showing
all saved versions with changed-field labels, word-level diff between
adjacent versions, and a compare mode for any two arbitrary versions.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/package.json

@@ -20,6 +20,7 @@
 		"generate:api": "openapi-typescript http://localhost:8080/v3/api-docs -o ./src/lib/generated/api.ts"
 	},
 	"dependencies": {
+		"diff": "^8.0.3",
 		"openapi-fetch": "^0.13.5"
 	},
 	"devDependencies": {
@@ -33,6 +34,7 @@
 		"@tailwindcss/forms": "^0.5.10",
 		"@tailwindcss/typography": "^0.5.19",
 		"@tailwindcss/vite": "^4.1.17",
+		"@types/diff": "^7.0.2",
 		"@types/node": "^24",
 		"@vitest/browser-playwright": "^4.0.10",
 		"eslint": "^9.39.1",