Redesign student UI and add OnlyOffice document integration

- Student section: new hero gradient header, fixed bottom nav bar with
  icon+label pills (Today/Courses/Spelling/Inbox/Docs)
- Today rows: card style with left accent stripe, lift on hover
- Course cards: grid layout with top color bar per course
- Docs tab: list view with doc cards, OnlyOffice editor embedded via
  iframe using server-signed JWT config
- Server: documents table, CRUD routes, OnlyOffice callback endpoint
  for auto-save, /api/docs/:id/editor-config returns signed config
- Blank DOCX template embedded as base64 for new document creation
- OnlyOffice server: https://onlyoffice.necloud.us (set ONLYOFFICE_JWT_SECRET
  env var to match your Document Server JWT secret)

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

db.js

@@ -244,6 +244,19 @@ tryAlter(`ALTER TABLE lists ADD COLUMN group_name TEXT NOT NULL DEFAULT ''`);
 tryAlter(`ALTER TABLE decks ADD COLUMN group_name TEXT NOT NULL DEFAULT ''`);
 tryAlter(`ALTER TABLE tests ADD COLUMN item_id INTEGER`);
 
+db.exec(`CREATE TABLE IF NOT EXISTS documents (
+  id INTEGER PRIMARY KEY,
+  student_id INTEGER NOT NULL REFERENCES students(id) ON DELETE CASCADE,
+  title TEXT NOT NULL DEFAULT 'Untitled Document',
+  doc_key TEXT NOT NULL UNIQUE,
+  filename TEXT NOT NULL,
+  submitted INTEGER NOT NULL DEFAULT 0,
+  submitted_at TEXT,
+  parent_read INTEGER NOT NULL DEFAULT 0,
+  created_at TEXT NOT NULL DEFAULT (datetime('now')),
+  updated_at TEXT NOT NULL DEFAULT (datetime('now'))
+)`);
+
 export const sha256 = (s) => createHash('sha256').update(String(s)).digest('hex');
 
 // Default parent PIN on first run