fix(trends): respect configured day boundary

[Charlie284]

Summary

Trends now anchor their date windows to the configured logical day instead of wall-clock midnight. Food totals, water buckets, chart windows, and streak calculations share the existing DayBoundaryCalc boundary and use calendar-date arithmetic across DST transitions.

Type of change

• Bug fix
• New feature
• Refactor / cleanup
• Documentation
• CI / tooling
• Localization
• Other

Related issues

Fixes #628

Changes

• Use the configured day-start offset when loading the current and prior trends windows.
• Bucket water entries into the same logical days used by food and diary data.
• Keep chart and streak date ranges aligned with logical today and local calendar dates across DST.
• Cover midnight and 08:00 boundaries, entries around midnight and the boundary, both aggregation paths, and a spring DST transition.

Screenshots / recordings

Not applicable; there are no visual changes.

Test plan

• Described steps below were followed locally
• Unit / widget tests added or updated (if applicable)
• Manual check on Android
• Manual check on iOS (if applicable)

Steps

1. Ran the focused day-boundary, trends, and refresh tests: passed, 52 tests.
2. Ran flutter analyze: passed with no issues.
3. Ran just test: passed, 966 tests.
4. Ran just format: completed. The pinned formatter also changes 259 files on an untouched develop checkout, so that unrelated repository-wide churn is not included here.
5. Ran just ci: stopped at its format --set-exit-if-changed gate for the same 259-file baseline mismatch; codegen, tests, and analysis were not reached by that recipe. The separate test and analysis commands above passed.

Checklist

• Code follows project style (just format / 120-char line width)
• No new interactive widgets were added
• No user-facing strings changed
• No DBOs, DTOs, or environment fields changed
• No secrets or .env values committed
• PR title follows conventional commit style (e.g. feat:, fix:, chore:)