add multialignment bar

[AniketR10]

What changed

Adds a floating alignment toolbar to the canvas (ThreadsView). When 2+ nodes are selected (Ctrl+click or Shift-drag box select), a small pill toolbar appears at the bottom-center of the canvas with 8 controls: align left / center / right, align top / middle / bottom, and distribute horizontally / vertically. Clicking a control recomputes each selected node's position from its real on-canvas size (width/height, falling back to React Flow's measured size or a default for auto-sized/unmeasured nodes), persists the new positions via saveThreadNode, and updates the canvas immediately — the same bulk-reposition pattern the existing "Tidy canvas" button already uses.

Also factored the dimW/dimH size-fallback helpers (previously local to tidy()) out to module scope so both features share one definition instead of duplicating the logic.

Related issue

Closes #16

How was this tested

• Ran the app locally with npm run dev on Linux.
• Created a thread, added several nodes (text/chat) via the top toolbar, dragged them apart.
• Multi-selected nodes via Ctrl+click and via Shift-drag marquee select — confirmed the alignment toolbar appears once 2+ nodes are selected and shows the correct "N selected" count.
• npx tsc --noEmit passes with no errors.

Checklist

• npx tsc --noEmit passes
• cargo check passes (if backend changed) — N/A, no backend files touched
• I've tested this on my OS — noted which: Linux (Fedora)
• Docs updated if user-facing behaviour changed
• No secrets, tokens, or personal paths in the diff

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