feat: add Sankey "Money Flow" chart to analysis tab

Three-column Sankey diagram (Income → Categories → Tags) rendered with
custom matplotlib bezier paths. Features gradient-fill bands that
interpolate color from source to destination nodes, neon glow effects
on nodes and bands, and a surplus node when income exceeds expenses.

Also fixes QFont::setPointSize warning by switching dark.qss font-size
units from px to pt, preventing Qt delegate code from receiving -1
point sizes during inline cell editing.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/plans/2026-02-10-sankey-money-flow-design.md

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+# Sankey "Money Flow" Chart — Design
+
+## Summary
+
+Add a new "Money Flow" tab to the Analysis view that renders a three-column Sankey diagram showing how income flows through spending categories into needs/wants/savings tags. Rendered entirely with matplotlib using custom bezier path drawing — no new dependencies.
+
+## Data Structure
+
+Three columns of nodes connected by curved bands:
+
+```
+Income ──► Categories ──► Tags
+```
+
+- **Left**: Single "Income" node — sum of all positive-amount transactions in the filtered period.
+- **Middle**: One node per spending category — sized by total absolute spend. Plus a "Surplus" node if income exceeds total expenses.
+- **Right**: Needs, Wants, Savings, Untagged — sized by total absolute spend per tag.
+
+Bands represent dollar amounts flowing between nodes. Thickness is proportional to amount. All values come from `AnalysisService` queries filtered by the standard `_FilterBar` (date range, account, person).
+
+## Visual Design
+
+### Theme Integration (Cyberpunk "Matrix 2026")
+
+- Background: `VOID` (#0a0a0f), matching all other charts
+- Axes styled via existing `_style_ax()` / `_style_figure()` helpers
+- Color palette: reuses `CHART_COLORS`, `TAG_COLORS`, `NEON_GREEN`, `NEON_AMBER`
+
+### Nodes
+
+- Thin vertical rectangles (width ~0.02 in axes coords)
+- Fill: node color at 60% opacity
+- Border: node color at full opacity, 1.5px
+- Glow: 2-3 progressively wider/more-transparent copies behind for neon bloom
+
+Colors:
+- Income node: `NEON_GREEN` (#00ff41)
+- Category nodes: `CHART_COLORS` palette (same as existing charts)
+- Tag nodes: `TAG_COLORS` (needs=green, wants=magenta, savings=cyan)
+- Surplus node: `NEON_AMBER` (#ffb627)
+- Untagged node: `TEXT_DIM` (#4a5568)
+
+### Bands (Flows)
+
+- Smooth cubic bezier curves (matplotlib `Path` with `CURVE4` codes)
+- **Gradient fill**: Each band sliced into ~50 vertical strips, color interpolated from source to destination node color
+- **Glow effect**: A wider, more-transparent copy of each band rendered behind it for neon bloom
+- Alpha: ~0.35 for main band, ~0.08-0.15 for glow passes
+- Sorting: Largest bands drawn first (back), smallest on top
+
+### Labels
+
+- Node name + dollar amount (e.g., "Groceries $1,234")
+- Left column labels: right-aligned, to the left of the node
+- Middle column labels: centered below/above nodes
+- Right column labels: left-aligned, to the right of the node
+- Font: `TEXT` color (#b8c4d4), fontsize 8
+- Dollar amounts: bold
+
+### Layout
+
+- Three columns at x = 0.1, 0.5, 0.9 (in axes normalized coords)
+- Nodes stacked vertically, sorted largest-on-top
+- Small gaps between nodes (~2% of total height)
+- Axes limits: x=[0, 1], y=[0, 1], all axes/spines hidden
+
+## Implementation
+
+### New Class: `_MoneyFlowTab(QWidget)`
+
+Follows the same pattern as `_SpendingOverTimeTab`, `_CategoryBreakdownTab`, and `_ForecastingTab`:
+
+```python
+class _MoneyFlowTab(QWidget):
+    def __init__(self, session, parent=None):
+        # _FilterBar, Figure, FigureCanvasQTAgg, NavigationToolbar2QT
+        # Connect filter signals to refresh()
+
+    def refresh(self):
+        # 1. Query income total, category totals, tag totals
+        # 2. Build node positions and sizes
+        # 3. Draw nodes, bands, labels via _draw_sankey()
+```
+
+### Helper: `_draw_sankey(ax, income, categories, tags)`
+
+Separated rendering logic for testability and clarity:
+
+1. Calculate node heights proportional to amounts
+2. Position nodes in three columns with vertical gaps
+3. For each flow (income→category, category→tag): draw gradient bezier bands with glow
+4. Draw node rectangles with glow
+5. Draw labels
+
+### Data Queries
+
+Reuses existing `AnalysisService` methods:
+- `spending_by_category()` — for category totals
+- `spending_by_tag()` — for tag totals
+- Direct query for income total (sum of positive amounts in filtered period)
+- Cross-query: category-to-tag amounts (new query needed — group by category_id + tag)
+
+### Tab Placement
+
+4th tab in `AnalysisView._tabs`: "Money Flow" — inserted after "Category Breakdown", before "Forecasting".
+
+## Files Changed
+
+- `src/ui/analysis_view.py` — add `_MoneyFlowTab` class and register in `AnalysisView.__init__`
+- `src/services/analysis.py` — add `spending_by_category_and_tag()` method for the category→tag cross-query
+
+## No New Dependencies
+
+Everything uses matplotlib's existing `PathPatch`, `Path`, `Rectangle`, and color utilities — all already available via the `matplotlib>=3.8` dependency.