feat: trade analysis equity overlays, theme settings, chart fixes

Trade Analysis:
- Add toggleable equity curve overlays: Account total, By Strategy,
  By Symbol, By Day of Week — checkboxes above each chart
- Fix all chart backgrounds to transparent (inherit page theme)
- Fix grid and font colours to work on both dark and light backgrounds

Settings page:
- Full theme control: Dark / Light / Custom radio with Apply button
- Custom mode: colour pickers for accent, page bg, sidebar bg, text,
  font selector and base dark/light toggle
- Preview swatches before applying
- Discard / Reset to Dark button
- Text size: Normal / Large (+2px) / Extra Large (+4px) via CSS injection
- inject_theme_css() helper for app.py to apply font size on all pages
- Theme written to .streamlit/config.toml on apply

app.py

@@ -9,6 +9,8 @@ Launch: streamlit run app.py
 import streamlit as st
 from streamlit_option_menu import option_menu
 import importlib, sys, os
+import view_settings
+view_settings.inject_theme_css()
 
 # ── Page config ───────────────────────────────────────────────────────────────
 st.set_page_config(