Alignment demo: neutral relationships, modal picker, gap overlay (#28)

* feat: merge toolbar into tab row, fix button styling, full LGAM alignment

- Combine view tabs and toolbar buttons onto single row (saves vertical space)
- Toolbar buttons hidden on Graph/About tabs
- Consistent button styling (outlined, accessible contrast)
- All LGAM layers now match reference exactly:
  Capabilities 9/9, FT 17/17, Integration 7/7, Security 7/7, Data 8/8

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* feat: add LGAM reference overlay toggle to graph view

Graph tab now has a "Show LGAM reference" toggle that overlays
all 86 canonical LGAM nodes (as faded ellipses) with dashed
alignment edges connecting council systems to the reference
items they map to. Toggles cleanly on/off.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* feat: neutral relationship language, modal picker, graph gaps toggle

- Replace directional "depends on" terminology with neutral
  "Related systems" / "Connected systems" throughout
- Remove graph edge arrows (connections not directed deps)
- Rework graph overlay to show only coverage gap nodes (dotted borders)
- Replace checkbox wall with layer-grouped modal picker for
  managing relationships (search, click to toggle)
- Merge toolbar into view tab row to save vertical space
- Consistent GOV.UK-aligned button styling

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: drhodesmumby-gds