fix: respect TZ env var as fallback for timezone config

[ItsAbdiOk]

Summary

Add TZ as a fallback in the env chain for both timezone and display_timezone in config/app.php, so the standard Docker TZ variable is respected when APP_TIMEZONE / DISPLAY_TIMEZONE are not set.

The problem

Docker users typically set timezone via TZ=Europe/London in their compose file. Currently, both config values fall back directly to 'UTC', ignoring TZ entirely. This causes times to display in UTC even when the container's system clock is correct.

This has been reported in #939, #1503, #805, #1739, and #1214 — all closed with "set APP_TIMEZONE and DISPLAY_TIMEZONE" as the workaround.

The fix

// Before
'timezone' => env('APP_TIMEZONE', 'UTC'),
'display_timezone' => env('DISPLAY_TIMEZONE', 'UTC'),

// After
'timezone' => env('APP_TIMEZONE', env('TZ', 'UTC')),
'display_timezone' => env('DISPLAY_TIMEZONE', env('TZ', 'UTC')),

Resolution order: APP_TIMEZONE → TZ → UTC

Non-breaking — existing deployments with APP_TIMEZONE or DISPLAY_TIMEZONE are unaffected.