Add table view for the conversation list (smaller-screen friendly)

[mageaustralia]

This branch is stacked on top of feat/bulk-actions (#286), so the diff currently shows both commits. If you merge #286 first I'll rebase this onto main and it'll shrink to just the table view changes. Or you can land them together as-is.

Picks up Peter2121's ask on #259 ("It would be nice to see Table view layout (like FreshDesk) integrated in mainstream").

What

A card/table view-mode switcher in the filter row of the conversation list. Card view is unchanged. Table view shows columns: Contact, Subject (with reference number and unread count badge), Status, Priority, Updated. Per-row checkboxes integrate with the bulk-action toolbar from #286. Click a row to open; click the checkbox cell to toggle selection (shift-click for range).

Columns are resizable: drag the right edge of any header (pointer events, so works for mouse, touch, and pen). Double-click resets that column. Widths persist per-browser via useStorage. View mode also persists and syncs across tabs.

Layout change (deliberate, bundled)

Worth flagging up front: the existing ResizablePanelGroup caps the conversation list at 45% width. A 6-column table at that width is unusable on a typical laptop. So when viewMode === 'table', InboxLayout.vue switches to a single-pane layout: full-width list when no conversation is open, full-width detail with a "Back to list" button when one is. Card mode is untouched.

I did consider splitting this into two PRs (table view inside the existing capped panel + layout-mode change separately), but the table feature is barely usable without the layout fix and reverting the two together is cleaner than reverting one without the other. Happy to split if you'd prefer.

i18n

10 new keys under conversation.list.*. All in alphabetical order in i18n/en.json. Crowdin will pick them up on merge.

Things I considered but left out

These each need their own PR and are not in scope here:

• Inline assign-agent / assign-team dropdowns per row (would need assigned_user_name / assigned_team_name exposed on the conversation list response)
• Status colored badges (would need a color field on the statuses schema)
• Subject hover preview tooltip
• Presence viewer indicator
• Keyboard a11y on resize handles (currently mouse/touch/pen only)
• A POST /api/v1/conversations/bulk endpoint (mentioned in Add bulk actions on conversations (assign, status, priority) #286)

Implementation notes

• Uses the shared-ui Table* primitives (TableHeader, TableBody, TableRow, TableHead, TableCell) for consistency with DataTable.vue. The shared Table wrapper itself isn't used because its built-in overflow-auto would create a nested scroll context; horizontal scroll lives on the parent content div instead, which puts the scrollbar at the viewport edge rather than mid-page.
• useViewMode and columnWidths use useStorage from @vueuse/core to match the rest of the persisted UI state in this codebase. localStorage keys are conversationViewMode and conversationTableColumnWidths.
• The route-building logic for opening a conversation is now in useConversationRoute. ConversationListItem was using an inline copy of the same logic; both now go through the composable.

Testing

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

• Switcher persists across reload and across tabs
• Column resize works via mouse and touchpad; double-click resets
• Click row to open, checkbox to select, shift-click for range
• Bulk action toolbar from Add bulk actions on conversations (assign, status, priority) #286 works identically in table mode
• Switching mode mid-conversation behaves correctly (split-pane resumes / single-pane resumes without losing the open conversation)
• Loading skeleton appears in both modes