feat(image-editor): Pen tool — vector paths with anchors and bezier handles

[lyfuci]

Summary

PS-aligned Pen tool, end-to-end:

• Click adds a corner anchor (no handles → straight segment).
• Click + drag turns the new anchor into a smooth one with symmetric handles (the drag delta from the anchor sets `hout`; `hin` mirrors).
• Click near the first anchor (within 8 px, ≥2 anchors total) closes the path.
• Enter commits the current path open.
• Esc cancels the in-progress path.

Result is a new annotation layer with a `path` shape — open paths render as a stroked curve; closed paths additionally support fill (the type carries `fill?`; the v1 OptionsBar exposes stroke + colour only — the fill swatch is a follow-up).

State + render layer

• types.ts: new `PathAnchor` (with relative `hin` / `hout` offsets) and `PathShape` join the Shape union. Persists through .json round-trip via the existing tagged-variant serialization.
• drawing.ts: `drawPath` walks anchors emitting `bezierCurveTo` / `quadraticCurveTo` / `lineTo` segments based on which handles each segment has on either side; closed paths get a final last→first segment.
• hit.ts: bbox covers anchor positions plus handle endpoints (rough bound — exact bezier extents are non-trivial; this is fine for layer picking). No corner handles in v1 — selection chrome shows the bbox.
• transform.ts: translate shifts anchor positions; relative handle offsets are unaffected.
• LayersPanel.tsx: `layerLabelKey` gains a `path` branch.

Canvas integration

• New `pen-drawing` interaction kind: `{ anchors, pressed, cursor }`.
• `mousedown` handles close-vs-append and sets `pressed=true`.
• `mousemove` with `pressed` updates the last anchor's symmetric handles from the drag delta. Without press, just tracks cursor for the rubber-band preview.
• `mouseup` releases `pressed`; anchors stay until close / Enter / Esc.
• `CanvasHandle` gains `hasPendingPen` / `commitPendingPen` / `cancelPendingPen`, mirroring the existing crop pair so Enter / Esc in the parent's keyboard handler route to the right action.
• `drawPenPreview` overlay: orange first-anchor square (close target), white squares for the rest; blue handle tethers + dots; dashed rubber-band line / curve from the last anchor to the cursor.

Other wiring

• ToolsPalette: pen entry no longer carries `stub: true`.
• tool-meta: `pen` removed from STUB_TOOLS — only the three sample-pixel tools (`spotHeal` / `stamp` / `historyBrush`) remain.
• OptionsBar: pen variant — stroke width + colour + hint text.
• i18n: `penHint` (en + zh-CN) and `annoLabel.path`.

Built on top of PR #45

This branch was cut from `feat/image-editor-note-frame-pathsel` (PR #45) since both tools touch the same files. Once #45 merges, this PR will be a clean fast-forward; if any conflicts arise on rebase, they'll be in the shape switch + STUB_TOOLS — easy to resolve.

Test plan

• P key → palette shows Pen active, cursor → crosshair.
• Click 3+ corners → straight-line path appears live; press Enter → committed as open Path layer.
• Click + drag → smooth anchor with visible blue handles; subsequent clicks continue.
• Click near the first orange-square anchor → path closes, layer named "Closed Path".
• Esc mid-path → in-progress curve disappears; no layer committed.
• Save .json → reload — open / closed path round-trips correctly.
• Move tool: drag a path layer — anchors translate together; handles stay relative.
• Crop applied → draw a path on the cropped image — coords align (no offset bug).

Stub palette status after this PR

Before: spotHeal, stamp, historyBrush, pen
After:  spotHeal, stamp, historyBrush

Final PR D (sample-pixel family — Spot Heal + Clone Stamp + History Brush) closes out the palette.

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