feat(terminal): drag & drop reorder for terminal slots

[iptoux]

Summary

• A drag handle (LuGripVertical) in every terminal titlebar lets you reorder entire slots (including split panes and agent labels) by dropping them on another slot in the same workspace grid.
• Reorder is a pure Vec permutation across slot_ids / slot_agent_labels / slot_pane_states → PTYs and xterm sessions are preserved (For key is slot.id, terminal_key stays stable).
• Drag state lives in TerminalSlotDragService (Leptos context); source and target are resolved live by slot_id against the workspace state (new WorkbenchService::terminal_slot_index), so repeated reorders compose without stale array indices.

Out of scope (V1)

• Cross-workspace transfer
• Dragging individual split panes out of a slot
• Custom HTML5 setDragImage (crashes WebKitGTK / Wry with both live and cloned DOM)

Implementation notes

• New module src/workbench/terminal_slot_dnd.rs — MIME application/x-blxcode-terminal-slot, payload helpers, drag service (non-reactive StoredValue<bool> session + generation guard against stale deferred updates), ghost-style helper.
• State (src/workbench/state.rs) — reorder_terminal_slots(workspace_id, from, to) + pure-Rust reorder_workspace_slots, plus terminal_slot_index.
• UI
  • src/workbench/terminal_cell.rs — grip handle on each slot, deferred try_set_active(gen, meta) so Leptos doesn't re-render mid-dragstart (which would otherwise cancel the drag on WebKitGTK).
  • src/workbench/workspace_panel.rs — dragover / drop handlers on the ws-term-slot wrapper, ghost overlay at the target cell, ws-term-grid--drag-active disables pointer events on foreign xterm canvases while a drag is in flight.
• CSS (styles.css) — .ws-term-cell__drag-handle, .ws-term-slot--drag-source (opacity 0.55), .ws-term-slot--drag-over (3px dashed outline), .ws-term-slot-ghost (2px dashed border, transluscent).
• i18n — new key WsTermDragHandleAria authored in all 13 locales.
• Tests — state::terminal_slot_tests (3 unit tests: parallel permutation, no-op on from == to, swap first slot).
• Docs — new section "Reordering terminals with drag & drop" in docs/user/workspaces.md; entry under [Unreleased] / Added in CHANGELOG.md.

Disabled states

• Workspace configurator is open
• Slot is in full-size mode
• Sidebar is collapsed

Test plan

• cargo check -p blxcode-ui --target wasm32-unknown-unknown clean
• cargo test -p blxcode-ui terminal_slot — 3/3 passing
• Manual (cargo tauri dev): 4 terminals running sleep 999 each → repeated reorders in both directions, PTY output keeps flowing, agent labels travel with the slot
• Manual: full-size slot → handle hidden, drag refused
• Manual: configurator open → no drag feedback
• Manual: reload after reorder → order persists