Interval date navigation + single-day drilldown (sub-month, 15-min)

[dalvikt]

Part of the AMI interval cluster (#76/#77). The dashboard's global RangeControl is month-resolution only (All / YTD / 12mo / 24mo / 36mo / custom-month, YYYYMM). 15-min electric data now exists, but there is no way to aim at it — you can't say "show me Tuesday June 10" or open one day at full 96-interval resolution.

Problem

• RangeControl / lib/range.ts speak only ym (YYYYMM); interval widgets receive a full-month [from, to] and the history chart just dumps a downsampled wide span.
• No day-level drilldown: no way to click into / select a single day and see its raw 15-min profile, nor to step day-by-day or week-by-week.
• For exploring intra-day structure (the whole point of 15-min data) the current UX is unusable.

Proposal

• Add sub-month navigation for the interval views: a day/week picker (and prev/next stepper) that resolves to a precise [from,to] day-span, independent of the month-grained global range. Keep lib/range.ts pure; add interval-range resolution as pure, hand-calc-tested helpers (don't compute in components).
• Add a single-day drilldown view: select a day (e.g. click a point in Usage history / a heatmap cell, or pick a date) → render that day's full 15-min (electric) / hourly (gas) profile. Reuse /api/interval (from/to already support a day span) and the existing self-fetching widget pattern; respect gaps as line breaks (never fabricated zeros, standards §1).
• Decide and document how the per-widget day selection relates to the global RangeControl (scoped override vs. global change) — keep it discoverable in the default view.

Acceptance criteria

• A user can navigate to and view a single day's interval data at native resolution (15-min electric, hourly gas).
• Day/week stepping (prev/next) and/or a date picker for the interval views.
• Range-resolution logic is pure and hand-calc unit-tested; gaps render as breaks.
• Works in the DEFAULT dashboard view (not hidden behind a non-default mode); cards hidden when the account has no interval data.
• /api/verify stays green (UI-only change); Docker test stage green.

Pointers

• components/RangeControl.tsx, lib/range.ts (month-grained today), components/Dashboard.tsx (range → ISO from/to threading), app/api/interval/route.ts (already accepts from/to day bounds), components/widgets/IntervalHistory.tsx + IntervalLoadShape.tsx (self-fetching pattern), lib/widgets/registry.tsx (isPlaced gating).