feat(fusion_accounting_reports): adopt Enterprise account_reports look

User feedback: 'i like the odoo enterprise style reports, I hate our style.'

Replaces our custom 'o_fusion_reports' visual with a faithful adaptation
of Enterprise account_reports. Same .account_report root class, same
table semantics (line_name + line_cell + line_level_N), same border
treatment (1px gray-300 borders, 0.25rem radius, sticky thead), same
button hover behavior (gray-300 -> enterprise-action-color), same dense
0.8rem font-size + padded cells.

SCSS layout:
- reports.scss in web.assets_backend bundle (eager light)
- reports.dark.scss in web.assets_web_dark bundle (lazy dark mode)
- _variables.scss reduced to spacing/typography only -- colors use
  Odoo's \$o-* SCSS vars so dark mode flips automatically via the
  separate dark bundle
- old dark_mode.scss removed (was using non-Odoo [data-color-scheme]
  selector that never matched anything)

QWeb templates rewritten to mirror Enterprise's structure:
- report_viewer.xml roots at .account_report with scroll container
- report_table.xml uses Enterprise's td.line_name + td.line_cell with
  .wrapper > .content nesting; partner-grouped reports now actually
  render their aging buckets (previously showed nothing)
- period_filter.xml is now a clean Bootstrap-styled inline filter bar

Kept Fusion-only components but restyled to fit:
- anomaly_strip uses Bootstrap alert-{danger,warning,info} colors
- ai_commentary_panel is a plain bordered panel, no gradients/emojis
- drill_down_dialog unchanged (already a Bootstrap modal)

Made-with: Cursor

fusion_accounting_reports/static/src/scss/_variables.scss

@@ -1,29 +1,15 @@
-// Fusion reports design tokens (extends Phase 1's bank_rec tokens for consistency).
+// Fusion reports design tokens.
+//
+// COLORS now come from Odoo's own SCSS palette ($o-view-background-color,
+// $o-gray-100..900, $o-enterprise-action-color). Dark-mode adjustments live
+// in the separate `reports.dark.scss` bundle (web.assets_web_dark) so they
+// load automatically when Odoo enters dark mode -- no [data-bs-theme] hack
+// is needed.
+//
+// This file therefore only carries spacing/typography tokens used by the
+// Fusion-only components (AI commentary panel, anomaly strip).
 
-// Colors — semantic
-$report-bg-primary:           #ffffff;
-$report-bg-secondary:         #f9fafb;
-$report-bg-tertiary:          #f3f4f6;
-$report-border:               #e5e7eb;
-$report-text-primary:         #111827;
-$report-text-secondary:       #6b7280;
-$report-text-muted:           #9ca3af;
-$report-accent:               #3b82f6;
-$report-accent-bg:            #eff6ff;
-
-// Severity colors (mirrors bank_rec)
-$report-severity-high:        #ef4444;
-$report-severity-high-bg:     #fef2f2;
-$report-severity-medium:      #f59e0b;
-$report-severity-medium-bg:   #fffbeb;
-$report-severity-low:         #10b981;
-$report-severity-low-bg:      #ecfdf5;
-
-// Variance indicators
-$report-variance-positive:    #10b981;
-$report-variance-negative:    #ef4444;
-
-// Spacing
+// Spacing scale (4px increments)
 $report-space-1:              0.25rem;
 $report-space-2:              0.5rem;
 $report-space-3:              0.75rem;
@@ -38,12 +24,7 @@ $report-font-size-sm:         0.875rem;
 $report-font-size-base:       1rem;
 $report-font-size-lg:         1.125rem;
 $report-font-size-xl:         1.25rem;
-$report-font-mono:            ui-monospace, SFMono-Regular, Menlo, monospace;
 
-// Borders + radii
-$report-border-radius:        0.375rem;
+// Border radius
+$report-border-radius:        0.25rem;
 $report-border-radius-md:     0.5rem;
-$report-border-radius-lg:     0.75rem;
-
-// Subtotal indentation
-$report-indent-per-level:     1.5rem;