Add custom theme colors, full per-token control (#30)

Adds a 5th "Custom" option to the theme swatch grid alongside the
existing 4 presets, opening a picker for all ~12 CSS custom properties
(backgrounds, borders, text, three accent tiers, highlight, danger) plus
a light/dark toggle for native control rendering.

Persisted as a new users.custom_theme_colors JSONB column, validated
server-side against exactly what a native <input type="color"> can ever
produce. Colors survive switching to a preset and back, since there's no
reason picking a preset for a moment should force redoing every color
pick. Applied at runtime as inline custom properties on :root (presets
stay static CSS) via a shared applyTheme() helper, which is also
responsible for clearing those inline overrides when switching away --
otherwise they'd silently keep winning over whatever preset's own
stylesheet values should apply next.

Live preview on every color change; persists only on explicit "Save
colors" (not per keystroke, since a color input fires continuously while
dragging), and closing the modal without saving reverts the preview back
to whatever's actually persisted.

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

backend/alembic/versions/3c04cf48f4b4_custom_theme_colors_for_user_profiles.py

@@ -0,0 +1,32 @@
+"""custom theme colors for user profiles
+
+Revision ID: 3c04cf48f4b4
+Revises: f9eff917e5b3
+Create Date: 2026-08-16 20:59:51.468679
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+
+# revision identifiers, used by Alembic.
+revision: str = '3c04cf48f4b4'
+down_revision: Union[str, Sequence[str], None] = 'f9eff917e5b3'
+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('custom_theme_colors', postgresql.JSONB(astext_type=sa.Text()), nullable=True))
+    # ### end Alembic commands ###
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_column('users', 'custom_theme_colors')
+    # ### end Alembic commands ###