Add split view to the Explorer (two panes side by side)

[jaysuk]

Summary

Adds a split view to the Explorer page: a pane can be split into two independent tab strips with a draggable divider between them, so two files (or a file and a directory listing) can be open side by side - e.g. reference one macro while editing another, or compare two config files.

• Split right button (in the toolbar, next to "+") creates a second pane and moves the active tab into it. Enabled once there are 2+ tabs to split from.
• Close split merges the second pane's tabs back into the first and removes the split.
• Each pane behaves exactly like the pre-split single tab strip: its own tabs, its own drag-and-drop reordering, its own "+" button.
• Tabs can be dragged from one pane's strip onto the other to move them across.
• The divider is draggable to resize the panes live; the ratio persists across sessions (cacheStore.explorerSplitRatio), mirroring the existing nav-drawer resize handle.
• Below the lg breakpoint the non-focused pane is hidden (not unmounted) and the split controls are hidden, so it degrades to the current single-pane behaviour on narrower viewports without losing state. (Originally gated on md; raised to lg since two side-by-side tab strips need more room than a typical tablet-width window gives them.)

Implementation notes

• Kept the existing flat tabs array and its URL-sync/drag/close logic largely intact by annotating each tab with a groupId (which pane it's in), rather than restructuring into per-pane arrays. groups is capped at exactly two fixed ids, so there's nothing to grow or clean up.
• setActiveTab() replaces direct activeTab.value writes and tracks which pane last had focus (focusedGroupId); the URL sync, scroll-restore watcher, and new-tab placement all key off it.
• moveTabToGroup() / splitRight() / closeSplit() handle pane transitions, reusing the existing closeTab fallback-tab-selection logic so closing/moving the active tab in a pane always lands on a sensible sibling.
• useGcodeThumbnails() is single-flight internally, so it's now instantiated once per pane rather than once for the whole page - otherwise two panes browsing different /gcodes directories would cancel each other's fetch and share one progress bar.
• The non-focused pane collapses via a CSS class on narrow viewports rather than v-if, since MonacoEditor's unmount has no unsaved-content flush path - unmounting a dirty editor would silently lose the edits.
• Browser back/forward ordinal resolution is scoped to the focused pane's own tabs, so a stray ordinal can never resolve into - and steal focus from - the other, non-focused pane.

Testing

• vue-tsc --noEmit and npm run build both pass, including after the md -> lg breakpoint change.
• Manually tested against a real Duet board: opened two macros side by side and dragged the divider to resize the panes.
• Also manually driven end-to-end against the dev server with Playwright (no live board): opened tabs, split, dragged the divider, dragged a tab across panes, closed the split, and shrank the viewport below the breakpoint - all confirmed via screenshots and DOM assertions, zero console/page errors throughout. That pass predates the md -> lg change; the collapse behaviour itself is unchanged, just the threshold it fires at, but it hasn't been re-run specifically against lg.
• Not yet exercised on real hardware: the narrow-viewport collapse, thumbnails against actual /gcodes directories in split panes, and the dirty-editor-survives-a-resize case.

🤖 Generated with Claude Code

[jaysuk]

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