feat: dashboard UI polish, metrics latency, import cleanup

[pratyush618]

Summary

• Comprehensive dashboard UI/UX polish pass covering design primitives, global chrome, all major pages, and two pre-existing API contract fixes
• Backend metrics endpoint now emits p50/p95/p99 latency per bucket alongside the existing avg
• 14 commits hoisting function-scoped imports to module level across the Python codebase, plus one module extraction to break a circular dependency

Changes

Design-system foundations

• StatCard now renders the previously-dead trend prop and gained a sparkline slot
• New TableSkeleton primitive replaces flat <Skeleton> blocks throughout
• useDeleteSetting now toasts on success

Global chrome

• Sidebar gains accent left-bar + aria-current="page" indicator
• Page-header title bumped to text-2xl
• Route-change fade-in re-fires on every navigation (was a one-time mount animation)
• Thin top progress bar tied to TanStack Query mutations (not poll cycles)

Overview homepage

• Stats grid shows trend deltas + mini sparklines for completed/failed (derived from existing throughput buckets, no new API)
• Throughput chart gains hover tooltip + edge time labels
• Plain-string empty states replaced with <EmptyState> components

Jobs list + detail

• Error column auto-hides when no row has an error
• Created column gains an absolute-time tooltip
• Table-shaped skeleton, tab content fades on switch
• Definition-list label column widened to 140 px

Metrics page

• metrics_timeseries backend endpoint now emits `p50_ms`/`p95_ms`/`p99_ms` per bucket (previously only `avg_ms`)
• Latency chart plots all four series with click-to-toggle legend + zoom brush
• Throughput chart gets a brush; metrics table groups columns into Volume/Latency sections

Secondary pages

• Worker heartbeat dot pulses when active
• System tables use `EmptyState` with icons; dead-letter rows highlight on hover
• Queues and circuit-breakers pages use `TableSkeleton`
• Queues and workers pages gain small stats banners

Bug fixes

• `/api/proxy-stats` type was `Record<string, ...>` but backend returns `list[dict]` — corrected
• `/api/interception-stats` type was `Record<strategy, {count, avg_ms}>` but backend returns totals + `strategy_counts` — corrected
• Recharts re-animated on every 2s poll cycle causing visible jitter — fixed with `isAnimationActive={false}`
• Top progress bar was strobing on every poll cycle — scoped to mutations only

Settings reorganization

• Refresh-interval toggle moved from the header into a dedicated card on the Settings page

Python inline-import cleanup

• 14 commits hoisting function-scoped imports to module top across `cli`, `app`, `task`, `testing`, `locks`, `mixins/inspection`, `prefork/child`, `proxies/reconstruct`, `resources/runtime`, all of `interception/`, `workflows/{builder,run,visualization}`, `async_support/{executor,mixins}`, and `contrib/{otel,flask,django}`
• Extracted `_ActiveContext` to `taskito/_active_context.py` to break the one legitimate circular import so `taskito.context` can import from `taskito.async_support.context` at module level

Test plan

• `pnpm --dir dashboard run ci` — Biome CI, typecheck, 97 Vitest tests, Vite build all green
• `uv run ruff check py_src/` — clean
• `uv run mypy py_src/taskito/ --no-incremental` — 0 issues across 121 files
• `uv run python -m pytest tests/python/` — 496 passed, 9 skipped
• `cargo check --workspace` — clean