The embedded spreadsheet component was updated to better handle range adjustments and chart data display. This helps users see cleaner charts based on available data and reduces errors when spreadsheet ranges change.

### Contains the following commits:

https://github.com/odoo/o-spreadsheet/commit/26a1856e [FIX] range: correctly adapt ranges
https://github.com/odoo/o-spreadsheet/commit/6db4c530 [IMP] chart: display only available data

The Planning app’s styling has been simplified and reorganized to make the calendar and schedule views easier to maintain. This should support a more consistent visual experience with low risk to day-to-day users, as the change is focused on presentation rather than business logic.

This PR simplifies and reorganizes the SCSS in the planning module.

task-2779458

Email theme templates were updated to use shared style definitions instead of repeating the same formatting throughout each template. This makes the themes easier to maintain and helps the design editor show complete, consistent styling information.

Global styling is possible since https://github.com/odoo/odoo/pull/86881
In this commit the inline styles used repetitively throughout the templates are moved to the style tag.

Odoo PR: https://github.com/odoo/odoo/pull/88318