refactor: restructure repo into core/app modules with per-study folders

Reorganize Palladium codebase into a modular architecture with `core/`
shared logic and `app/` Streamlit UI, separating per-study assets into
`studies/YYYYMM_<Vendor>/` folders containing notebooks, seed data, and
configuration. Update README to reflect new structure, add `.gitignore`
entries for `.env` and study exports, and refresh component documentation.

app/pages/costs.py

@@ -0,0 +1,46 @@
+"""Costs data-entry tab."""
+
+from __future__ import annotations
+
+import streamlit as st
+
+from app.components.tables import df_to_values, value_editor
+from app.pages._helpers import report_meta, safe
+from core.tei_client import TEIClient
+
+
+def render(client: TEIClient, tool: dict) -> None:
+    st.header("💸 Costs")
+    public_id = tool["id"]
+    report = report_meta(client, tool)
+    analysis_years = int(report.get("analysis_period_years") or 3)
+
+    fields = safe(client.list_fields, report.get("id"), "costs") or []
+    values = [v for v in safe(client.get_values, public_id) or [] if v.get("table") == "costs"]
+
+    if not fields:
+        st.info("This report template has no cost fields defined.")
+        return
+
+    edited = value_editor(
+        "costs",
+        fields,
+        values,
+        analysis_years=analysis_years,
+        key=f"costs_editor_{public_id}",
+    )
+
+    col1, col2 = st.columns([1, 4])
+    with col1:
+        if st.button("💾 Save costs", use_container_width=True):
+            payload = df_to_values(edited, "costs", analysis_years)
+            result = safe(client.update_values, public_id, payload)
+            if result is not None:
+                st.success(f"Saved {len(payload)} cost values.")
+                st.cache_data.clear()
+    with col2:
+        st.caption(
+            "The Initial column is undiscounted year-0 spend. Year columns "
+            "are end-of-year cashflows. Costs are risk-adjusted upward "
+            "(higher risk → higher cost)."
+        )