Support explicit start/end date range controls

[raychrisgdp]

Problem

The dashboard currently centers range selection around last N days presets plus a custom numeric days box. That is fast, but it does not let users select an explicit date window such as 2026-06-01 → 2026-06-15.

We should support explicit start/end dates while keeping the current quick presets.

Desired UX

Add date range controls that support both workflows:

• quick presets: 3d, 7d, 30d, 90d, All
• custom numeric days: N days
• explicit date range: Start date and End date

Behavior:

• Selecting a preset fills the date inputs automatically.
  • Example: 7d sets start/end to the computed last-7-days window.
• Entering N in the custom days box also fills start/end automatically.
  • Example: 45 sets start = today - 44 days, end = today using the dashboard’s current local-date convention.
• Editing either start or end date switches the active mode to explicit date range.
• Applying explicit dates updates overview, daily chart, model breakdown, and history consistently.
• If start/end are invalid, show an inline error and do not fetch.
• If All is selected, clear or disable explicit date inputs with clear copy.

Implementation Notes

Current APIs are days-based. We need to decide the backend contract before changing UI behavior.

Suggested simple contract:

• Keep existing days support for presets and backwards compatibility.
• Add optional query params to relevant endpoints:
  • start_date=YYYY-MM-DD
  • end_date=YYYY-MM-DD
• If start_date/end_date are provided, they take precedence over days.
• Use one shared date-window parser/helper so overview/models/daily/history stay consistent.
• Keep source provenance and token semantics unchanged.

Likely endpoints affected:

• /api/overview
• /api/models
• /api/daily
• /api/usage-history

Likely frontend changes:

• templates/index.html
• range-control state management
• fetch URL construction
• active button/explicit-range state
• accessible labels for date inputs

Acceptance Criteria

• Users can select an explicit start and end date from the homepage controls.
• Preset buttons still work as today.
• Entering custom N days automatically populates start/end.
• Editing start/end switches to explicit range mode.
• All dashboard sections use the same resolved date window.
• Invalid date ranges show a visible inline error.
• Existing days query behavior remains supported.
• Tests cover:
  • default range behavior
  • custom days filling start/end
  • explicit start/end precedence over days
  • invalid date range rejection
  • all affected endpoints using consistent boundaries.