Fast mode for Dashboards and Discover

**What the feature is (as Title)**
A new toggle lets users switch between accuracy and speed when loading dashboards and Discover sessions. When Fast Mode is enabled, queries use approximation techniques behind the scenes to return results significantly faster.

**Value proposition**
Not every analysis requires exact numbers. When users are exploring data, scanning for trends, or monitoring high-level patterns, waiting for precise results across large datasets slows them down. Fast Mode gives users control over that tradeoff — they get near-instant results when speed matters, and can switch back to full accuracy when precision is critical.

**Expected outcome**
- A visible toggle in both Dashboards and Discover allows users to switch between standard and fast mode.
- In fast mode, panels and queries load faster by using approximate aggregations under the hood.
- Users can switch modes at any time without losing their current view or configuration.

**Optional: Key user stories / use cases**

No files, tests, or implementation entry points are identified in the issue, so begin by locating the Dashboards and Discover loading and query paths in the relevant project. Done means both views expose a mode toggle, fast mode uses approximate aggregations, and switching modes preserves the current view and configuration.

Written by the indexing model from the issue text.