Implement Inter as primary sans in Dataface charts

Problem

Apply Inter as the primary sans-serif typeface for Dataface chart and data-dense UI surfaces, including numeric defaults, fallback behavior, and render/export consistency.

Context

• Dataface has chosen Inter as its primary sans-serif typeface for UI and chart surfaces.
• The rationale is documented in ai_notes/considerations/INTER_AS_PRIMARY_SANS.md.
• The implementation target is: Inter everywhere unless a theme intentionally opts out.
• Opinionated themes and structures should not be rewritten just to force Inter.
• Relevant implementation seams already exist:
• dataface/core/render/chart/presentation.py centralizes chart font family access.
• dataface/core/compile/chart_defaults.yml carries Vega/Vega-Lite font defaults.
• dataface/core/render/chart/kpi.py, spark_bar.py, and table.py consume the shared chart font family.
• dataface/core/render/templates/controls/_styles.css already uses font-variant-numeric: tabular-nums for some control surfaces.
• Rendering/export compatibility matters:
• Vega/Vega-Lite charts render through vl-convert.
• SVG-to-PNG/PDF export uses svglib and reportlab.
• This task should implement the product default, not just document the decision.
• Numeric policy for this task:
• Use tabular-nums lining-nums for most numeric chart/UI surfaces, especially tables, accounting layouts, numeric controls, multi-value tooltips, axes, tick labels, numeric legends, timestamps, metadata rows, and other dense numeric displays.
• Keep KPIs as proportional lining numerals by default because they are usually standalone display values rather than vertically aligned numeric columns.
• Narrative numbers in prose and annotation can remain proportional unless alignment pressure makes tabular figures clearly better.

Possible Solutions

• Recommended: Set Inter as the default sans-serif family through the shared chart/style configuration, vendor/register the font centrally for renderers, and tighten numeric defaults where aligned values need tabular figures. Trade-offs: delivers the chosen visual language through the main rendering path and keeps typographic behavior centralized, but requires careful validation across SVG, HTML, and export surfaces.
• Change only a few visible templates or CSS files without updating shared defaults. Trade-offs: faster initially, but risks inconsistent typography between chart internals, KPI/table renderers, and exported outputs.
• Leave global defaults untouched and require explicit per-face font overrides. Trade-offs: lowest implementation risk, but defeats the goal of a coherent product-level type system.

Plan

• Audit the current default font-family path across chart config, render helpers, and exported surfaces.
• Update shared defaults so Inter becomes the default sans-serif family for chart and data-dense UI surfaces unless a theme intentionally sets its own font choices.
• Vendor/register Inter so the render pipeline can resolve it consistently.
• Apply the numeric policy for this task:
• tabular lining numerals for most numeric chart/UI surfaces
• proportional lining numerals for KPIs by default
• Verify the change in rendered examples and exports so Inter appears consistently without breaking fallback behavior.
• Confirm rendering behavior across SVG, HTML, PNG, and PDF paths.
• Document any remaining follow-ups separately if broader typography controls are still needed.

Implementation Progress

• Task created after the primary sans-serif decision was completed.
• Initial codebase scan found the main font-family integration points in compile defaults, chart presentation helpers, render templates, and converter paths.
• Numeric direction is now explicitly defined: broad tabular usage for dense numeric surfaces, but KPI displays remain proportional by default.
• Vendored InterVariable.ttf under the render package so chart/export code does not depend on host-installed fonts.
• Switched shared default typography config from Liberation Sans to Inter in both default_config.yml and chart_defaults.yml.
• Added central font registration for vl-convert and moved SVG-to-PNG/PDF export onto vl-convert so SVG, Vega, PNG, and PDF all share the same font-aware conversion path.
• Updated remaining hardcoded system-font render paths in HTML wrapper, variable controls, placeholder overlays, chart tooltips, timestamps, and spark/table numeric surfaces.
• Added focused tests covering default config, rendered SVG/HTML output, tooltip and placeholder font pickup, and Inter-backed SVG export to PNG/PDF.

QA Exploration

• Visual review completed on rendered example faces via the local Dataface server:
• /faces/sales_dashboard
• /faces/product_performance
• /faces/customer_analytics
• /faces/general/analytics-dashboard
• Visual check outcome: approved; Inter looked satisfactory across charts, tables, controls, and supporting UI text.
• [x] QA exploration completed (or N/A for non-UI tasks)

Review Feedback

• [ ] Review cleared