feat(ui): multi-graph mosaic dashboard with persistent layout

[fedorovvvv]

Replace single-canvas-with-view-toggle home page with a recursive binary split-tree mosaic that hosts 1–4 DependencyGraph panes. Each pane has its own view selector, reset, add and close controls in a draggable header; layout (open panes + sizes in %) persists to localStorage under forgeplan-web:layout:v1.

Drag semantics:

• Pane header onto another pane's center → swap views (sizes kept).
• Pane header onto another pane's edge → move (remove source +
  insert at target's edge).
• Splitter pointer/keyboard drag → resize, clamped [10..90]%.

Layout model lives under template/src/widgets/mosaic/ (FSD widget):

• model/{types,tree}.ts pure tree ops with vitest coverage (20 tests).
• lib/{drag,persist}.ts module-level drag payload (workaround for the spec-mandated empty getData during dragover) + 5 quadrant tests.
• ui/{MosaicCanvas,MosaicNodeView,PaneFrame,Splitter,DropOverlay}.svelte recursive render via 3-track CSS Grid (a% 0px b% with overlay splitter) — no JS layout math at render time.

A11y: splitters expose role=separator with arrow-key resize, pane headers are toolbar with tabindex=-1, drag fallback writes text/plain so the operation initialises in browsers that require non-empty data.

Refs: PRD-016, RFC-015, EVID-020