perf(treemap): on-demand subtree fetch + visible directory labels

[aksOps]

Summary

Two related improvements to the dashboard treemap, building on the depth-8 cap shipped in v0.2.1:

Phase 2 — backend path-rooted subtree

Adds an optional `path` query parameter to `GET /api/file-tree` so the frontend can fetch a focused subtree instead of the whole tree. Cypher filters via `STARTS WITH path/`, the tree builder strips the prefix during walk so the local root represents the focused directory, and `@Cacheable` is keyed on path so different anchors are cached separately. Path inputs are normalized + validated in the controller (no `..`, no leading slash, ≤1024 chars).

Phase 3 — frontend on-demand drill

When the user clicks a directory rendered as a depth-cap leaf (type=directory, no children, nodeCount > 0), Dashboard fetches its subtree via the new path endpoint, splices the response into the in-memory tree at that node, and drills into it in the same gesture. A small loading indicator appears in the breadcrumb during the fetch. Concurrent clicks dedupe via a ref-tracked in-flight set; failures clear the flag so retries work.

UX fix — labels on directory cells

The design-system Treemap canvas only paints names on `isLeaf` cells (line 114 of its canvas paint), so directories at the visible drill level showed as unlabelled rectangles — names only appeared on hover. Dashboard now feeds the treemap a render-only flat copy of the focused level: directories are stripped of their children and given a pre-aggregated value so the canvas treats them as leaves and paints the label. The original TreemapNode (with intact children) stays in `focusedRoot` and is recovered via a `renderToOriginal` WeakMap in the click handler so drill-down keeps working.

Test plan

• `mvn test -Dfrontend.skip=true` — 3706 / 3706 pass, 32 expected skips
• `npm run build` — 197.47 kB / 63.02 kB gzip (+0.5 kB vs main)
• 4 new `GraphControllerTest` cases: path passthrough, trailing-slash normalization, `..` traversal → 400, overlong path → 400
• Deploy + verify treemap shows directory names by default
• Manual: drill into a depth-8 truncated directory, observe subtree-loading indicator, confirm subtree appears and drill continues

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