Add time range warning in query editor for large time ranges (#2150)

## Summary

Adds a non-intrusive warning banner in the query editor that alerts
users when the selected time range exceeds 1 year (365 days). This helps
users understand that their query may return a large amount of data and
could take longer to execute, without blocking or interrupting their
workflow.

Part of https://github.com/grafana/oss-big-tent-squad/issues/127

## Changes

- Added `TIME_RANGE_WARNING_THRESHOLD_DAYS` constant (365 days) in
`src/datasource/constants.ts`
- Created new `TimeRangeWarning` component in
`src/datasource/components/TimeRangeWarning.tsx`
- Integrated the warning component into the main `QueryEditor` component

## How it works

- When the dashboard time range is >= 365 days, a warning banner appears
at the top of the query editor
- The warning displays the formatted duration (e.g., "1 year and 30
days")
- The warning is purely informational - queries still execute normally
- Uses Grafana theme colors for consistent styling in both light and
dark modes

## Screenshot

The warning appears as a subtle banner with a warning icon:

<img width="705" height="374" alt="grafik"
src="https://github.com/user-attachments/assets/eb0ace4b-524a-488e-8f88-b7e9523660b0"
/>

## Why

Queries spanning years of data can return millions of data points and
potentially overload the Zabbix server. This proactive warning helps
users make informed decisions about their query scope without adding
friction to the normal query flow.

src/datasource/constants.ts

@@ -51,3 +51,5 @@ export const RANGE_VARIABLE_VALUE = 'range_series';
 export const DEFAULT_ZABBIX_PROBLEMS_LIMIT = 1001;
 
 export const ITEM_COUNT_WARNING_THRESHOLD = 500;
+
+export const TIME_RANGE_WARNING_THRESHOLD_DAYS = 365;