Fix timezone handling: use configured timezone instead of browser timezone

[fatherlinux]

Summary

• Fixes bug where pomodoro entries appeared in wrong AM/PM sections when browser timezone differed from configured app timezone
• Adds global appTimezone variable that reflects the user's configured timezone setting
• Adds "Automatic" option to timezone dropdown showing browser-detected timezone
• Introduces createDateInTimezone() helper for timezone-aware date creation
• Adds automated Puppeteer tests for timezone functionality

Changes

• Add appTimezone global variable updated when settings load or timezone changes
• Add createDateInTimezone() to interpret user-entered times in configured timezone
• Add formatTimeSnappedTz() and formatDateWithDayTz() timezone-aware helpers
• Update formatDate() and formatDateWithDay() to use appTimezone
• Fix AM/PM split in weekly grid to use configured timezone
• Fix addManualPomodoro() and addPomoToNextSlot() to create entries in configured timezone
• Add tests/test_timezone.js with automated Puppeteer tests

Test plan

• Set browser to one timezone (e.g., Europe/Brussels)
• Set app timezone to a different timezone (e.g., America/New_York)
• Add entry via PM + button - verify it appears in PM section
• Add entry via timer start/stop - verify time displays correctly
• Set app timezone back to match browser - verify entries still display correctly
• Test "Automatic" timezone option shows detected browser timezone

Run automated tests: node tests/test_timezone.js

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