fix(dashboard): fix Home savings 'All' range and label chart tooltip date

[cristim]

Closes #1248

Symptoms

• QA 3.1: Clicking the "All" timeframe button on the Home page Savings-over-time chart shows "Savings history is not available yet." while 7d/30d/90d render correctly.
• QA 3.2: Hovering over a data point shows an unlabeled 13-digit millisecond timestamp as the tooltip header instead of a human-readable date.

Root cause

QA 3.1: loadSavingsTrendChart sent start=1970-01-01T00:00:00Z (an epoch sentinel) when the "All" range was selected. The backend analytics handler (internal/api/handler_analytics.go) caps the date range at 366 days and returns HTTP 400 "date range too large" for a 1970 start. The catch block renders the generic empty-state message for all errors, so the 400 surfaced as "Savings history is not available yet."

QA 3.2: The chart x-axis is type: 'linear' with data points {x: timestampMs, y: value}. The tooltip had a label callback but no title callback, so Chart.js rendered the raw x value (milliseconds) as the tooltip header.

Fix

• QA 3.1: Replace the 1970 epoch sentinel with a rolling ~365-day window (now - 365 * 86400_000) that fits inside the backend's 366-day cap. The existing axisMinMs anchor-to-earliest-point logic is preserved so sparse data still fills the chart width correctly. The backend 366-day cap (a deliberate DoS guard) is unchanged.
• QA 3.2: Add a title callback to the tooltip that formats the x timestamp using the existing formatTrendAxisTick helper (already used for x-axis tick labels).

Testing

• Replaced the old "epoch sentinel" test (which asserted the broken behavior) with a test asserting the "All" range sends a start within ~365 days of now (not 1970), and that the chart renders rather than showing the empty-state when the API returns data.
• Added a test asserting the tooltip title callback returns a formatted date string (not a bare 13-digit number) for a given x timestamp.
• Both new tests confirmed to FAIL on pre-fix code and PASS after the fix.
• npx jest src/__tests__/dashboard.test.ts: 97 pass, 0 fail, 1 skipped.
• npx tsc --noEmit: no errors.

---

Stacked on #1254 (pre-commit repair); retarget to main when #1254 merges.

Summary by CodeRabbit

• Bug Fixes
  • Updated the savings trend chart’s “All” range to query analytics using a rolling ~365-day window instead of an unbounded epoch-based start.
  • Improved the savings trend chart tooltip header to display human-readable dates rather than raw timestamps.
• Tests
  • Updated dashboard chart tests to match the new “All” range start behavior and added coverage for tooltip date formatting.

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Adversarial-review pass

Ran the playbook against the QA 3.1 / QA 3.2 fixes plus the explicit risk surfaces (All semantics, stale docstring memory hit, tooltip date format, empty/zero state, timezone, range cross-product, tests).

In-scope fixes pushed (8008bc0)

1. Stale docstring on loadSavingsTrendChart (frontend/src/dashboard.ts:1004-1010). The docstring still described the pre-QA-2.3 behaviour ("Empty windows render labelled axes rather than a 'no data' stub"); the shipped code (lines 1083-1098) shows a filter-aware empty-state banner instead. This was a tracked TODO in the project memory garden (memory todo_dashboard_trend_docstring), held until the next edit to the file. This PR is that edit -- updated the docstring to describe the actual three branches (data / empty / fetch-error). The memory entry is now stale and can be deleted.
2. Redundant mock setup in the new All-range test (__tests__/dashboard.test.ts:887-893). The test called mockResolvedValue twice with a mockClear() between -- mockClear() resets call history but NOT mock return values, so the first set was dead code. Collapsed to one mockResolvedValue + mockClear() with a clarifying comment. No behaviour change; 97 dashboard tests still pass.

Confirmed clean

• 365-day window fits the backend 366-day cap: windowStartMs = now - 365 * 86400_000, backend cap is < 366 * 24h (handler_analytics.go:309-315). 1-day cushion.
• Tooltip title callback: reuses formatTrendAxisTick, has nullish-guard on raw?.x so missing-x renders '' rather than "NaN" or the raw timestamp.
• Empty-state path on 'All': already covered by the generic data_points: [] test (line 920); the same points.length === 0 branch handles all ranges.
• Fetch-error path: existing test at line 978 still covers the catch block.
• Tests confirmed pre-fix-FAIL / post-fix-PASS: 97/97 dashboard tests pass on this branch; tsc --noEmit clean.

Out-of-scope follow-ups filed

• fix(dashboard): Home 'All' range toggle truncates history at 365 days but is labeled 'All' #1288 -- the toggle button is labeled "All" but actually returns a rolling 365-day window. Accounts with purchases older than 365 days are silently truncated. Filed as separate fix because the resolution (rename button, paginate, or relax backend cap) is a UX/product decision, not a side effect of the QA 3.1 patch.
• fix(dashboard): savings-trend tooltip omits year for daily/weekly intervals; ambiguous across year boundaries #1289 -- the tooltip date format (shared with axis ticks) omits the year. For the 90d (daily) and new All (weekly) ranges, two points labelled "Jun 15" from different years are indistinguishable. Proposed fix: separate formatTrendTooltipTitle helper that includes the year (axis-tick stays compact).

Risk surfaces from the brief, checked

• Timezone (same risk as fix(ui): enforce future-only planned start date + widen Configure Purchase Term field #1258): formatTrendAxisTick uses toLocaleString in the user's local timezone while the backend buckets in UTC. This PR doesn't change that behaviour -- it inherits the existing handling. Out of scope here; tracked under the broader fix(ui): enforce future-only planned start date + widen Configure Purchase Term field #1258 follow-ups.
• $NaN / zero state: cumulative_savings || 0 and raw?.x != null guards prevent NaN tooltip values; the empty-state banner replaces the canvas before any tooltip can render.
• Range cross-product (7 / 30 / 90 / All): interval bucketing (hourly | daily | weekly) gives sensible point density on all four; tooltip formatter handles all three intervals.

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