### Unassigned Booking Lines

A booking line can now have no resource linked to it, representing guests that will attend but for whom we could not find an available resource. These bookings will appear in the gantt view as a pill in the "unassigned" row so users know they need to do something about that booking.

Additionally, a warning is shown at the top of the form in such cases.

### Automatic resource assignment

For bookings linked to appointment types that are set as "automatically assign", when the user modifies the "reserved capacity" of an entire event, available resources will automatically be suggested.

The suggestion is based around groups of linked resources, attempting to assign resources from the largest groups using the fewest possible resources. This superficially matches what the website would do when automatic assignment is set.

If the requested capacity is still too high, a confirmation dialog will appear before saving to let the user know they are exceeding what the system would normally allow. In which case an empty booking line will be created to cover the overcapacity.

task-6185278

Asking the AI agent to navigate to a different view calls doAction with clearBreadcrumbs, which tears down the current form view (and its chatter) before the still-open full mail composer dialog is closed. Closing that dialog then calls the chatter's onCloseFullComposerCallback, which reloads the parent record through reloadParentView(). However at that point the chatter no longer exists, so the reload's RPC call gets rejected with "Component is destroyed", and nothing is left to catch it.

How to reproduce:
- Open "Ask AI" from the top bar.
- Open an opportunity in CRM, then open the mail composer (click either 'Send message' or 'Log Note'), then click the enlarge button so it opens as a full composer dialog.
- Ask the agent (the one you pre-opened) to open another view, e.g. "show me all my contacts in the US", "show me the contact view of Abigail Peterson"

Current behavior:
The requested view opens correctly, but an UncaughtPromiseError ("Component is destroyed") is raised.

Expected behavior:
Switching views while a full composer dialog is open should not raise any error. The chatter's parent record should simply not be reloaded if the chatter has already been destroyed by the time its full composer dialog closes.

task: 6365003


---
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Forward-Port-Of: odoo/odoo#287877
Forward-Port-Of: odoo/odoo#287125

**Steps to reproduce:**
- Install `im_livechat` module
- Copy the code from the livechat channel's widget tab
- Paste it in an external website `<head>` (e.g., local python webserver on `0.0.0.0`)
- Start a conversation and send a file from odoo
- Try to download it from the external website
- POST request is sent for the download
- Download fails: `CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource.`
- Also when the file is a PDF the preview won't open: `404 (File not found)`

**Issue:**
Mix of multiple issues:
- The download is triggered using a POST request instead of a GET request
(see similar issue for the file viewer [1])
- The file route does not provide the required CORS headers, so requests
originating from the external website are blocked by the browser
- PDF files are rooted to the related path of the `pdfjs` fileviewer
(e.g. `http://0.0.0.0:8000/web/static/lib/pdfjs/web/viewer.html?file=...`)

```xml
<!--
    Template rendering all the scripts required to execute the Livechat from an external page (which not contain Odoo)
-->
<template id="external_loader" name="Livechat : external_script field of livechat channel">
    <!-- the loader -->
    <script defer="defer" t-attf-src="{{url}}/im_livechat/loader/{{channel_id}}" type="text/javascript"/>
    <!-- js of all the required lib (internal and external) -->
    <script defer="defer" t-attf-src="{{url}}/im_livechat/assets_embed.js" type="text/javascript" />
</template>
```

**Fix:**
Make the `downloadFile` helper handle cross-origin URLs by falling back to a native `<a download>` click (like before) when the target route has the same origin as the embedded script. Could use `session.origin` or `new URL(document.currentScript.src).origin` for this. This is done to avoid allowing CORS on the file content route.

The file viewer issue is handled separately by [1].

For the PDF issue we could manually add the origin to the full url everywhere (but we get some `SecurityError` error from the library due to the cross-origin iframe), add the libjs library in the `assets_embed` (not sure it's possible in `im_livechat.assets_embed_external`), or block external PDF preview for now.

[1] https://github.com/odoo/odoo/pull/281330

opw-6444167

Forward-Port-Of: odoo/odoo#287846
Forward-Port-Of: odoo/odoo#286180