docs: per-tool pages for the vector-canvas toolbar

Adds a new "Vector tools" group to the in-app docs sidebar with one
page per toolbar tool + one page per supporting panel:

Toolbar tools (8):
- tool-select.md           — Select (V): move/scale/rotate whole layers,
                             multi-select, marquee, dive-into-path
- tool-direct-select.md    — Direct Selection (A): per-anchor edits,
                             handle break, segment-direct-edit
- tool-hand.md             — Hand (H): pan + temp-pan via Spacebar,
                             plus zoom controls (which work from any
                             tool)
- tool-pen.md              — Pen (P): full Illustrator parity —
                             Bézier draw, mid-draw spacebar reposition,
                             Alt handle-break, auto-add/delete anchors,
                             Cmd+J join, Cmd+Alt+J average
- tool-pencil.md           — Pencil (B): freehand → smoothed Bézier
- tool-shape.md            — Shape (S): rect/ellipse/polygon/star/line
                             primitives + Shift/Alt modifiers
- tool-text.md             — Text (T): rich-text layer + Type panel
                             + token notes
- tool-eyedropper.md       — Eyedropper (I): window.EyeDropper sample +
                             inline ⊙ buttons + browser-support notes

Supporting panels (3):
- pathfinder.md            — Unite/Subtract/Intersect/Exclude via the
                             vendored polygon-clipping
- align-distribute.md      — six align ops + four distribute ops, with
                             behaviour around locked layers
- free-transform.md        — multi-layer scale + rotate handles on
                             the selection bbox

Plus an overview page (vector-tools.md) with a keyed table of every
shortcut, a description of how the toolbar interacts with the Layers
panel, and the canvas-level keyboard shortcuts (Shift/Alt/arrow
nudges/Cmd+D/etc).

All linked into _DOCS_TOPICS as a new "Vector tools" group, ordered
after the existing Template-mode layouts section.

Co-Authored-By: Claude Opus 4.7 <[email protected]>

src/help/tool-shape.md

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+# Shape tool (S)
+
+Drops vector primitives — rectangle, ellipse, polygon, star, line.
+The right-side **Tool options** pane picks the primitive + per-shape
+parameters before you drag.
+
+## Primitives
+
+- **Rectangle** — drag a bounding box.
+- **Ellipse** — drag a bounding box; the ellipse inscribes it.
+- **Polygon** — N-sided regular polygon. The options pane sets the
+  side count (3 – 32) before you drag.
+- **Star** — M-pointed star. Options pane sets points + inner-radius
+  ratio.
+- **Line** — drag from start to end.
+
+## Drag modifiers
+
+- **Shift** — constrain to a perfect square / circle, snap polygon
+  rotation to 15°, lock line to 45° angles.
+- **Alt** — draw from the centre outward instead of corner-to-corner.
+
+## Why these are vector
+
+Every shape is stored as a Bézier path under the hood. Switching to
+**Direct Selection** on any of them auto-converts the primitive into
+an editable anchors-plus-handles path. You can then add / remove /
+modify anchors like any other path.
+
+## Fill + stroke
+
+The right-side **Fill** and **Stroke** panels apply at draw time:
+
+- Solid colour, gradient, or "none".
+- Stroke width, dash pattern, line cap.
+- The active settings render live in the preview while you drag.
+
+## Tips
+
+- For non-rectangular text containers, draw a shape, then add a text
+  layer on top + use the layer's **Clip** option to clip the text
+  to the shape.
+- The **line** primitive's endpoints are anchors — they show up under
+  Direct Selection and can be moved independently.