fix(#553): unify Dashboard-tree row counts and move Add panel to the Panels row

[BorisTyshkevich]

What & why

Closes #553. Dashboard tree rows used inconsistent columns for counts and actions: the Dashboard row's panel count was right-aligned trailing text competing with the action cluster, while Variables and Panels used an inline · N. At narrow sidebar widths that right-aligned metadata ate the space the Dashboard and group names needed.

Now all three rows use one placement, and the rightmost column is actions only:

On-time flights · 7   … edit  delete
  Variables · 4
  Panels · 7          … +

The change is in the pure model, not the renderer. src/ui/dashboard-tree.ts renders row.count / row.actions generically, so both halves flowed through src/application/dashboard-tree-model.ts with no render change:

• the Dashboard row's panel count moves from meta (right-aligned) into count (inline), and its meta is now always '';
• add-panel moves off the Dashboard row's actions onto the Panels group row, because it creates a member of that group rather than acting on the Dashboard document. A new addPanelAction helper preserves the availability rules exactly — an ambiguous Dashboard id or the 100-tile ceiling still render the control disabled with a reason rather than withholding it (Dashboard tree: replace overflow menus with direct Panel focus, edit and delete actions (#429 phase 4) #494's "vocabulary must not silently shrink"). The Variables group row still has no actions.

The accessible name is provably unchanged. rowAccessibleName composes [label, count, status, meta, marker] and drops empties, so a Dashboard row that was count: null, meta: '7' → "Sales revenue 7" is now count: 7, meta: '' → the same string. Hiding the count at the breakpoint is purely visual: the row's aria-label is set explicitly, so a screen reader still announces the count. There's an explicit e2e assertion for that at ≤220px.

Narrow width reuses #552's breakpoint

One line added to the existing @container sidebar (max-width: 220px) block from #552 — .dash-tree-count { display: none; }. No second container, no second threshold, no new class.

Verification

npm test → 201 files / 6630 tests, dashboard-tree-model.ts and ui/dashboard-tree.ts both clear the per-file floor. npm run build and npm run check:arch pass.

New e2e coverage: a Dashboard tree counts at the narrow sidebar (#553) block (3 tests) plus a Panels-row keyboard/Tab-order test, covering the narrow state, the restored wider state, and that expand/collapse and row actions stay usable by mouse and keyboard at both widths. tests/e2e/tile-open-workbench.spec.js's Add-panel flow is retargeted to the Panels row.

Verified visually in Chromium at 1280px and dragged to 200px: counts disappear, a long dashboard title visibly gains the recovered space, and Panels' + stays present and reachable.

Note on two unrelated e2e failures

tests/e2e/tile-open-workbench.spec.js has 2 failures (grid-tile widen width/height) that reproduce on the base branch — confirmed by stashing this branch's changes and re-running on a private port, after checking via /proc/<pid>/cwd that the shared Playwright port was held by another worktree (see #560). They are pre-existing and outside this issue's scope; filed separately rather than folded in here.

Checklist

• npm test passes (the per-file coverage gate is non-negotiable)
• Tests added/updated in the same change as the code
• npm run build succeeds (single-file dist/sql.html)
• Layers kept honest: pure logic in src/core/, network in src/net/ (injected fetch), DOM in src/ui/
• No new runtime dependency
• README / CHANGELOG.md ([Unreleased]) updated if behavior or the deployed surface changed
• Reconciled affected tracked work (roadmap Roadmap to 1.0.0 #68, the issue body, ADR/CHANGELOG) if this change reshaped it

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