fix(visualization): use #graph-svg selector instead of bare svg

[itxaiohanglover]

Summary

• d3.select("svg") returns the first <svg> in document order, which is a 16px legend icon — not the main #graph-svg canvas. This caused the entire force graph to render inside the tiny legend swatch while the full-screen canvas stayed empty.
• Fix: target #graph-svg by id in both template copies (full mode + aggregated/community mode).
• Added id="graph-svg" to the aggregated-view template's <svg> element (it was missing).

Root cause

In visualization.py, the legend <nav> is emitted before #graph-svg and its first child is an inline <svg> icon:

<svg width="16" height="16" viewBox="-8 -8 16 16" aria-hidden="true">...</svg>

d3.select("svg") matches this 16px icon, so gRoot + all nodes/links get appended into it (viewBox -8 -8 16 16), producing a microscopic graph in the legend and an empty canvas.

Fix

// Before
var svg = d3.select("svg").attr("viewBox", [0, 0, W, H]);

// After
var svg = d3.select("#graph-svg").attr("viewBox", [0, 0, W, H]);

Applied to both template copies. The aggregated-view template's <svg> also gets id="graph-svg" added (it was missing).

Test plan

• test_generate_html_uses_id_selector_for_svg — verifies full-mode HTML uses d3.select("#graph-svg") and contains no bare d3.select("svg")
• test_community_mode_uses_id_selector_for_svg — verifies community-mode HTML uses d3.select("#graph-svg"), has no bare d3.select("svg"), and the <svg> has id="graph-svg"
• All 22 existing visualization tests still pass
• ruff check passes

Closes #523