feat: WeChat Pay integration, translation quota management, login UX fixes

- WeChat Pay APIv3 integration (JSAPI + Native) with cert-based auth
- TranslationQuota model + admin management UI (配额 tab)
- Alibaba MT provider now checks quota before translation
- Fix: admin tabs scrollable on mobile, remove header-card
- Fix: profile/login navigation - logout stays on profile, login returns to profile
- Fix: login form now visible by default (no extra click to show)
- Fix: home page translate link uses navigateTo (was switchTab to non-tabBar page)
- Add .coverage and apiclient_key.pem to gitignore

backend/alembic/versions/93a81b22bd80_add_translation_quota_model.py

@@ -0,0 +1,37 @@
+"""add translation_quota model
+
+Revision ID: 93a81b22bd80
+Revises: eee1d35c9b88
+Create Date: 2026-05-20 15:43:47.299785
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+
+revision: str = '93a81b22bd80'
+down_revision: Union[str, None] = 'eee1d35c9b88'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table('translation_quotas',
+    sa.Column('id', postgresql.UUID(as_uuid=True), nullable=False),
+    sa.Column('version', sa.String(length=50), nullable=False),
+    sa.Column('monthly_limit', sa.Integer(), nullable=False),
+    sa.Column('used_chars', sa.Integer(), nullable=False),
+    sa.Column('current_month', sa.String(length=7), nullable=False),
+    sa.Column('enabled', sa.Boolean(), nullable=False),
+    sa.Column('description', sa.Text(), nullable=True),
+    sa.Column('updated_at', sa.DateTime(), nullable=True),
+    sa.PrimaryKeyConstraint('id')
+    )
+    op.create_index(op.f('ix_translation_quotas_version'), 'translation_quotas', ['version'], unique=True)
+
+
+def downgrade() -> None:
+    op.drop_index(op.f('ix_translation_quotas_version'), table_name='translation_quotas')
+    op.drop_table('translation_quotas')