Add bulk actions on conversations (assign, status, priority)

[mageaustralia]

Adds bulk actions on conversations, per the item you mentioned in #259. Focused PR as you suggested there.

What

Per-row checkboxes on the conversation list. When at least one conversation is selected, the filters bar swaps for a small toolbar with three dropdowns: Assign (agent or team), Status, Priority. Shift-click selects a range. The X on the right clears.

How

Frontend only. The bulk operations call the existing per-conversation API endpoints (updateAssignee, updateConversationStatus, updateConversationPriority) in parallel via Promise.allSettled. No new backend handler.

That keeps the diff small. Trade-offs worth flagging:

• 50 selected items = 50 concurrent HTTP requests. Wall-clock for a typical bulk op is sub-second on a fast network, but it's not ideal at scale.
• No transactionality. If some calls fail, you get a toast like "Updated 47, failed 3 of 50" and the per-uuid errors are logged to console.

Permissions

Each dropdown is gated on the corresponding existing permission:

• Assign Agents: `conversations:update_user_assignee`
• Assign Teams: `conversations:update_team_assignee`
• Status: `conversations:update_status`
• Priority: `conversations:update_priority`

A user with none of these still sees the count and clear button (since they could already select rows).

i18n

8 new keys under `conversation.bulkActions.*` in `i18n/en.json`. Reused `globals.terms.{agent,team,status,priority,error}` for the singular and plural labels, so the Crowdin export is small. Other locales fall back to `en` until translated.

Testing

Tested manually against a cloned production dataset (~5k conversations, ~80k contacts). Verified:

• Single selection, multi-selection, shift-click range
• Bulk assign agent, bulk assign team
• Bulk change status, bulk change priority
• Clear selection, including after a partial-failure run
• Toolbar shows the correctly localized count (singular vs plural)
• Permission gating verified by removing perms from a test role

[abhinavxd]

Hey thanks for the PR! This is amazing and is exactly what we needed at work.

I tried it locally few suggestions -

• Place the checkbox on top of the avatar and show it on hover. This saves space and removes the need for a permanently visible checkbox on the left.

• The top options could be just icons to save space.

We can also allow changing the assigned team separately using the SelectCombobox component, since it supports search filter.
We could also allow setting tags.

Overall: assign team, assign agent, set tags, and status - all with icons to save space, with tooltips on each icon.
We can drop priority; not having it will space space.