This PR fixes issues after the revamp in PR https://github.com/odoo/odoo/pull/138670

The first fix was to move some of the SCSS in the `calendar` module into the `web` module. Because some modules such as `planning` don't require `calendar`, if it isn't installed, `planning` would not have the styling and colors of the events. By moving things around, this makes for a major part of the diff in this PR.

Some event styles were also not being applied in the Year view. This is because we moved all the styles for event styling inside the `.fc-event` selector, but the Year view doesn't have this class on its events, instead we changed it to the `o_event` class which is in every view.

This PR includes other improvements such as making the events' colors stand out more in Year view, making "mandatory days" more readable, and improving the calendar header's layout. 

Entreprise PR https://github.com/odoo/enterprise/pull/50251

task-3575827

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Currently, the customer, who did not perform the operation, is set 
as the author of the refund, capture, and void transaction messages.

**Steps to reproduce:**
- Install the `sale_management` module and enable the `demo` payment provider.
- Create a new quotation and from the `gear` icon, select `Mark Quotation as Sent`.
- From the `gear` icon, choose `Generate a Payment Link`, and copy the link.
- Open the link in an incognito window, and complete the payment.
- Return to the original tab and open the payment transaction from the chatter.
- Click `Refund` > `Refund` and observe the `author` of the messages in the chatter.

**Observation:**
The refund messages entry shows the customer as the author instead of the 
logged-in user who executed the refund.

**Root Cause:**
At [1] and [2], when the logged-in user is not a superuser, the code assigns 
the customer (`self.partner_id`) as the author, even during `refund`, `capture`, 
and `void transactions`, resulting in an incorrect message author.

**Fix:**
This commit ensures that during refund, capture, and void transactions, 
the message author is set to the logged-in user instead of the customer.

[1]:
https://github.com/odoo/odoo/blob/bbf3bd7b0e1d3e015350f9c959fad056384e2318/addons/account_payment/models/payment_transaction.py#L223

[2]:
https://github.com/odoo/odoo/blob/5cf325dfff1ffbc1c4b4d0bcfc44ac9f55ea7c51/addons/sale/models/payment_transaction.py#L110

**Before:**
<img width="1910" height="624" alt="5379376_before" src="https://github.com/user-attachments/assets/decdd620-d741-488e-9dec-e5c8c9bafc48" />

**After:**
<img width="1917" height="622" alt="5379376_after" src="https://github.com/user-attachments/assets/5f38f3c3-fd2c-489e-afc9-d7a356fde23a" />

opw-5379376

### Issue:
When a custom button snippet was dropped next to an existing button 
wrapped in a `<div>`, it was incorrectly and unconditionally wrapped in 
a paragraph `<p>` tag. This issue did not occur with standard button 
snippets or when the buttons were wrapped in a `<p>` tag.

### Steps to reproduce:
1. Drag a standard "Button" snippet next to an existing button (e.g., in
   the navbar). The structure is correctly `<a/><a/>`.
2. Save a button as a custom snippet.
3. Drag this new custom button snippet next to a button wrapped in a 
   `<div>` (navbar in our example).
4. Observe the structure: 
   `<div><a/><p><a class="s_custom_snippet"/></p></div>`,
   but it should be like `<div><a/><a class="s_custom_snippet"/></div>`.

### Fix
The logic has been corrected to only wrap a button snippet in a 
paragraph tag if it has no sibling buttons and is not already inside a 
`<p>`. This ensures consistent behaviour between standard and 
custom button snippets. A tour test is also added, ensuring the 
behaviour is correct.

Backport of https://github.com/odoo/odoo/commit/3a8f2cc418510c3b91f8b9086bb4a89a1745e40d

task-[5032130](https://www.odoo.com/odoo/project/974/tasks/5032130)