[FIX] side_panel: prevent chart picker layout shift on hover

[fw-bot]

Description:

Current behavior before PR:

• Hovering or selecting a chart type was adding a border while reducing padding to compensate for the size change.
• At non-100% zoom levels, fractional pixel rounding caused inconsistent sizing, leading to layout shifts in the flex container.
• This resulted in chart icons 'dancing' when moving the cursor between them.

Desired behavior after PR is merged:

• A transparent border is applied in the default state to reserve space.
• On hover/selection, only the border color is updated without changing padding.
• This ensures consistent element dimensions and prevents layout shifts across all zoom levels.

Task: 6095239

review checklist

• feature is organized in plugin, or UI components
• support of duplicate sheet (deep copy)
• in model/core: ranges are Range object, and can be adapted (adaptRanges)
• in model/UI: ranges are strings (to show the user)
• undo-able commands (uses this.history.update)
• multiuser-able commands (has inverse commands and transformations where needed)
• new/updated/removed commands are documented
• exportable in excel
• translations (_t("qmsdf %s", abc))
• unit tested
• clean commented code
• track breaking changes
• doc is rebuild (npm run doc)
• status is correct in Odoo

Forward-Port-Of: #8307

[robodoo]

[fw-bot]

This PR targets 19.0 and is part of the forward-port chain. Further PRs will be created up to master.

More info at https://github.com/odoo/odoo/wiki/Mergebot#forward-port

[fw-bot]

@rmbh-odoo @hokolomopo the next pull request (#8430) is in conflict. You can merge the chain up to here by saying

@robodoo r+

More info at https://github.com/odoo/odoo/wiki/Mergebot#forward-port

[rmbh-odoo]

@robodoo r+