feat(frontend): settings window, i18n, admin UI, books, tasks, calendar, timer

- Settings window with tabs: profile (name, email, avatar, password),
  appearance (light/dark/system), language (de/en/zh), background
  (gradient presets, custom URL, blur toggle); persisted server-side
- i18n dictionaries (de/en/zh) applied across nav, sidebar, tasks,
  calendar, time tracking, admin, invites and comments
- Greeting header with time-of-day message, live clock and date; cooler
  theme, cards, kanban, calendar grid and data-table styling
- Shared sidebar: nav, collapsible books with per-book pages, top-level
  pages, search; user chip opens settings
- Tasks kanban with priorities, overdue flags and book/page links;
  calendar month view fed by task deadlines; time view with entries,
  durations and totals; top-right live timer with optional book link
- User discovery autocomplete in the invite card; editor gains tables
  and checklists; standalone theme buttons removed

frontend/package-lock.json

@@ -9,6 +9,12 @@
       "version": "0.1.0",
       "dependencies": {
         "@tiptap/core": "^2.4.0",
+        "@tiptap/extension-table": "^2.4.0",
+        "@tiptap/extension-table-cell": "^2.4.0",
+        "@tiptap/extension-table-header": "^2.4.0",
+        "@tiptap/extension-table-row": "^2.4.0",
+        "@tiptap/extension-task-item": "^2.4.0",
+        "@tiptap/extension-task-list": "^2.4.0",
         "@tiptap/pm": "^2.4.0",
         "@tiptap/starter-kit": "^2.4.0",
         "@tiptap/vue-3": "^2.4.0",
@@ -1128,6 +1134,86 @@
         "@tiptap/core": "^2.7.0"
       }
     },
+    "node_modules/@tiptap/extension-table": {
+      "version": "2.27.3",
+      "resolved": "https://registry.npmjs.org/@tiptap/extension-table/-/extension-table-2.27.3.tgz",
+      "integrity": "sha512-xBDaT/ixkmrHKuCUlJNbzRolxjf0fFkHxybyeqfcGNDbVwOb6pOGZly6mGjBG1TXrx3U37y0jmFpwW6WU1zokA==",
+      "license": "MIT",
+      "funding": {
+        "type": "github",
+        "url": "https://github.com/sponsors/ueberdosis"
+      },
+      "peerDependencies": {
+        "@tiptap/core": "^2.7.0",
+        "@tiptap/pm": "^2.7.0"
+      }
+    },
+    "node_modules/@tiptap/extension-table-cell": {
+      "version": "2.27.3",
+      "resolved": "https://registry.npmjs.org/@tiptap/extension-table-cell/-/extension-table-cell-2.27.3.tgz",
+      "integrity": "sha512-f697x9RDiva8EFZvLBxEp6Nx6sY5KRbHdNMhbCnqC1PrkRKNCTOib9ddWpqIERzs3HhmqZWL7XCPzvE/t8C+IQ==",
+      "license": "MIT",
+      "funding": {
+        "type": "github",
+        "url": "https://github.com/sponsors/ueberdosis"
+      },
+      "peerDependencies": {
+        "@tiptap/core": "^2.7.0"
+      }
+    },
+    "node_modules/@tiptap/extension-table-header": {
+      "version": "2.27.3",
+      "resolved": "https://registry.npmjs.org/@tiptap/extension-table-header/-/extension-table-header-2.27.3.tgz",
+      "integrity": "sha512-x//GuYJlTzM3GavsDxBRDjGHMY9bbnmglQ0EAPVwQM5QHXpjwKzZN7wvDMM/RwxmxzYWd6Z+Fp7Zk8RYckpzhg==",
+      "license": "MIT",
+      "funding": {
+        "type": "github",
+        "url": "https://github.com/sponsors/ueberdosis"
+      },
+      "peerDependencies": {
+        "@tiptap/core": "^2.7.0"
+      }
+    },
+    "node_modules/@tiptap/extension-table-row": {
+      "version": "2.27.3",
+      "resolved": "https://registry.npmjs.org/@tiptap/extension-table-row/-/extension-table-row-2.27.3.tgz",
+      "integrity": "sha512-0ekXiw6aAESlcrKFmI90ar/j7XJmfdxeJEDeGZuMwOAQs+qq5RAB7ug5WPCZjG0Kqa+Kyxzb1jMw9pSkuTYp+w==",
+      "license": "MIT",
+      "funding": {
+        "type": "github",
+        "url": "https://github.com/sponsors/ueberdosis"
+      },
+      "peerDependencies": {
+        "@tiptap/core": "^2.7.0"
+      }
+    },
+    "node_modules/@tiptap/extension-task-item": {
+      "version": "2.27.3",
+      "resolved": "https://registry.npmjs.org/@tiptap/extension-task-item/-/extension-task-item-2.27.3.tgz",
+      "integrity": "sha512-7ES+qMoMgjb2iXI8iRzgegMeM1pE4zQAekT/5lQyuTv6oEyZw5s3TUZWeTYCq5TI/MezYGu3X2sV5PQnfBAsdA==",
+      "license": "MIT",
+      "funding": {
+        "type": "github",
+        "url": "https://github.com/sponsors/ueberdosis"
+      },
+      "peerDependencies": {
+        "@tiptap/core": "^2.7.0",
+        "@tiptap/pm": "^2.7.0"
+      }
+    },
+    "node_modules/@tiptap/extension-task-list": {
+      "version": "2.27.3",
+      "resolved": "https://registry.npmjs.org/@tiptap/extension-task-list/-/extension-task-list-2.27.3.tgz",
+      "integrity": "sha512-T8Y3S95ZzYlh82KGd0H81QOsJZVWweUVps6l7wkiOReNsJxhkb4qW/smNE0TeNhvbU307emiPL0CH5e+eHxjEg==",
+      "license": "MIT",
+      "funding": {
+        "type": "github",
+        "url": "https://github.com/sponsors/ueberdosis"
+      },
+      "peerDependencies": {
+        "@tiptap/core": "^2.7.0"
+      }
+    },
     "node_modules/@tiptap/extension-text": {
       "version": "2.27.3",
       "resolved": "https://registry.npmjs.org/@tiptap/extension-text/-/extension-text-2.27.3.tgz",