fix(#294): dashboard filter strips never wrap, remove visible Clear-all

[BorisTyshkevich]

What & why

Closes #294. PR #293 (Dashboard v1 phases 1-4) shipped a visible Clear all
filter control and an "N active" status that wrapped onto extra toolbar rows
once several {name:Type} filters were present at desktop widths, growing the
toolbar and shrinking the dashboard's data area. This reverses that specific
UX decision (owner call, per #294): filters now behave like editor tabs and
the existing Workbench variable strip — one stable row, horizontal scroll for
overflow, at every viewport width (not just the ≤768px breakpoint from
#248) — with no visible Clear-all control.

Changes

• src/ui/filter-bar.ts: deleted filterClearAllButton (no remaining UI
  consumer). filterActiveCount (the "N active" status) is unchanged.
• src/ui/dashboard.ts: filter host is now .dash-filter-host >
  (.filter-strip-scroll wrapping the shared buildFilterBar output, +
  .dash-filter-count-host as a fixed sibling). No more clearAllNode.
  DashboardViewerSession.clearAllFilters() is untouched — it stays a tested
  application-level operation with no UI trigger.
• src/styles.css:
  • .dash-toolbar is flex-wrap: nowrap at every width (was wrap-capable
    on desktop, nowrap only under the mobile media query).
  • New .filter-strip-scroll viewport (nowrap/overflow-x:auto/hidden
    scrollbar) holds the fields; .filter-strip-scroll .dash-filters is
    scoped to nowrap (not the base .dash-filters class, which the detached
    Data view's filter row also uses and still wraps there — commented in
    place).
  • The "hide native scrollbar, auto horizontal scroll" contract is now one
    shared rule read by both the Workbench .var-strip and the new
    .filter-strip-scroll, instead of two copy-pasted blocks (review
    follow-up — the issue explicitly asked for a shared utility here).
  • Two fixes that came out of an independent multi-angle review pass before
    this PR: the scroll viewport now carries enough vertical padding that a
    focused field's box-shadow focus ring isn't clipped by its
    overflow-y: hidden, and the "N active" count is align-self: center so
    it stays vertically centered against the field row even when a field
    (e.g. relative-time's resolved-value preview line) is taller than one
    line — both are covered by new real-browser regression assertions
    (verified to fail without the fix, pass with it).
• Tests: updated tests/unit/dashboard.test.ts (no more Clear-all
  assertions; new coverage for the separate scroll/count DOM regions and for
  "no Clear-all at any active count"). Extended
  tests/e2e/dashboard-mobile.spec.js with a desktop-width test (one row,
  toolbar height stable, count/layout-switcher fixed and aligned, no
  Clear-all, no page overflow, focus-ring padding) and a taller relative-time
  fixture field. New tests/e2e/workbench-var-strip.{html,spec.js} pins the
  Workbench .var-strip's pre-existing never-wrap/scroll contract in a real
  browser (it was already correct — CSS wrapping/overflow is invisible to the
  happy-dom unit suite, so this generalizes the acceptance requirement to a
  real-browser check for both surfaces).
• CHANGELOG.md [Unreleased] and roadmap Roadmap to 1.0.0 #68's Dashboard v1 phase 4 (#280): DashboardViewerSession + normative flow@1 layout #286 bullet updated to
  record the reversal.

Out-of-scope findings filed separately (not fixed here)

Both predate this issue, surfaced by an independent review pass:

• Dashboard filter fields have no accessible group name #295 — the production Dashboard filter row has no accessible group name
  (buildFilterBar is called without ariaLabel, unlike the detached view).
• Combobox popover doesn't reposition on horizontal filter-strip scroll (broader exposure after #294) #296 — the shared combobox popover doesn't reposition on scroll (a
  documented v1 limitation); this PR broadens where it's reachable (every
  width, not just mobile) without introducing the underlying gap.

Checklist

• npm test passes (135 files / 3361 tests; per-file coverage gate holds)
• 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
• Reconciled affected tracked work (roadmap Roadmap to 1.0.0 #68's Dashboard v1 phase 4 (#280): DashboardViewerSession + normative flow@1 layout #286 bullet, this issue's acceptance criteria)
• npm run test:e2e (chromium + webkit, 86 specs) passes
• /code-review (high effort, 8-angle fan-out + verification) run; findings applied

Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_01GLMprUnwnz4oaz3cnLRKcz