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/src/lib/generated/api.ts

@@ -308,6 +308,38 @@ export interface paths {
         patch?: never;
         trace?: never;
     };
+    "/api/documents/{id}/versions": {
+        parameters: {
+            query?: never;
+            header?: never;
+            path?: never;
+            cookie?: never;
+        };
+        get: operations["getVersions"];
+        put?: never;
+        post?: never;
+        delete?: never;
+        options?: never;
+        head?: never;
+        patch?: never;
+        trace?: never;
+    };
+    "/api/documents/{id}/versions/{versionId}": {
+        parameters: {
+            query?: never;
+            header?: never;
+            path?: never;
+            cookie?: never;
+        };
+        get: operations["getVersion"];
+        put?: never;
+        post?: never;
+        delete?: never;
+        options?: never;
+        head?: never;
+        patch?: never;
+        trace?: never;
+    };
     "/api/documents/{id}/file": {
         parameters: {
             query?: never;
@@ -516,6 +548,27 @@ export interface components {
             /** Format: date-time */
             startedAt?: string;
         };
+        DocumentVersionSummary: {
+            /** Format: uuid */
+            id: string;
+            /** Format: date-time */
+            savedAt: string;
+            editorName: string;
+            changedFields: string[];
+        };
+        DocumentVersion: {
+            /** Format: uuid */
+            id: string;
+            /** Format: uuid */
+            documentId: string;
+            /** Format: date-time */
+            savedAt: string;
+            /** Format: uuid */
+            editorId?: string;
+            editorName: string;
+            snapshot: string;
+            changedFields: string;
+        };
     };
     responses: never;
     parameters: never;
@@ -1189,6 +1242,51 @@ export interface operations {
             };
         };
     };
+    getVersions: {
+        parameters: {
+            query?: never;
+            header?: never;
+            path: {
+                id: string;
+            };
+            cookie?: never;
+        };
+        requestBody?: never;
+        responses: {
+            /** @description OK */
+            200: {
+                headers: {
+                    [name: string]: unknown;
+                };
+                content: {
+                    "*/*": components["schemas"]["DocumentVersionSummary"][];
+                };
+            };
+        };
+    };
+    getVersion: {
+        parameters: {
+            query?: never;
+            header?: never;
+            path: {
+                id: string;
+                versionId: string;
+            };
+            cookie?: never;
+        };
+        requestBody?: never;
+        responses: {
+            /** @description OK */
+            200: {
+                headers: {
+                    [name: string]: unknown;
+                };
+                content: {
+                    "*/*": components["schemas"]["DocumentVersion"];
+                };
+            };
+        };
+    };
     getDocumentFile: {
         parameters: {
             query?: never;