Rework the Discuss sidebar to match the new design. The tab navigation becomes a vertical icon rail on every viewport (it was a bottom bar on mobile), the "New" action and the search box share a first row, and the tab filters render as pill toggles on a second row.

Channel rows are restyled with rounded corners, an action-tinted hover/active state and an accent bar on the active row, replacing the global contrast() filter that read too dark on the light surface. The three surfaces now form a consistent elevation hierarchy in both light and dark mode: the rail is deepest (a faint pastel of the brand primary), the channel list sits above it, and the thread is lightest.

Subchannel rows abbreviate the parent channel to its first few letters so the subthread name stays the prominent label.

Channels without a custom image now get an avatar generated from the name initials over a stable pastel color derived from the record, instead of the fixed monochrome SVG.

task-6392371

https://github.com/odoo/odoo/pull/276874