Add adjustable text size and emoji size preferences (#71)

Text was too small on high-DPI screens with no in-app fix beyond
browser zoom. Adds a text-size setting (scales the whole app via a
root font-size percentage), auto-large rendering for emoji-only
messages, and an independent emoji-size preference that also scales
reaction pills without affecting the emoji picker's fixed-size grid.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/alembic/versions/e81c9bcc82b9_add_user_emoji_scale_preference.py

@@ -0,0 +1,32 @@
+"""add user emoji_scale preference
+
+Revision ID: e81c9bcc82b9
+Revises: 339b78011a4f
+Create Date: 2026-08-30 18:14:26.045186
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'e81c9bcc82b9'
+down_revision: Union[str, Sequence[str], None] = '339b78011a4f'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.add_column('users', sa.Column('emoji_scale', sa.String(length=20), nullable=True))
+    # ### end Alembic commands ###
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_column('users', 'emoji_scale')
+    # ### end Alembic commands ###