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/views/menu_views.xml

@@ -6,7 +6,7 @@
         sitting alongside Community's PDF-based "Statement Reports" /
         "Partner Reports" / "Taxes & Fiscal" / "Management" wrappers.
 
-        Each menu opens an act_window with view_mode='fusion_reports'
+        Each menu opens an ir.actions.client with tag='fusion_reports'
         (the OWL ReportViewer). report_actions.xml defines the actions.
 
         All gated to the coexistence group so they only appear when