feat: add DIN rail (TH35) mount option

[vZhurbenko]

Summary

Closes #37

Adds a parametric TH35 top-hat rail mount as a second, independent toggle inside the existing Wall Mounts section (renamed to Mounting). The mount is a slotted boss that protrudes from the chosen enclosure face with a trapezoidal undercut channel cut through it, so a standard 35 mm top-hat rail slides in end-on and is retained by the narrower mouth.

The existing wallMounts flag is left untouched — wall flanges and the DIN boss are independent and can be active simultaneously.

What is new

• src/app/core/enclosure/dinrail.ts — new geometry module (channel + boss + placement).
• src/app/core/enclosure/dinrail.spec.ts — 7 Jasmine specs covering defaults, protrusion, placement (bottom/back), channel cuts volume, coexistence with wall mounts, and undercut retention.
• src/app/core/params.ts — 6 new params (dinRailMount, dinRailSurface, dinRailWidth, dinRailCrownWidth, dinRailDepth, dinRailLength) + TH35 defaults.
• src/app/core/enclosure/base.ts — extra if (params.dinRailMount) branch alongside the existing wallMounts branch.
• src/app/features/renderer/renderer.component.ts — the 6 DIN params added to baseDeps.
• src/app/features/params/params-form.component.{html,ts,css} — accordion renamed to Mounting; new DIN toggle group (Surface select, Rail Width, Crown Width, Rail Depth, Slot Length number inputs); checkboxes forced onto their own full-width row.
• src/app/core/state/enclosure-state.service.ts — resetToSimpleEnclosure now also resets dinRailMount.
• README.md — feature ticked off.

Design notes

• TH35 defaults: brim/base width 35, crown/top width 27, height 8.5. All four numbers are exposed as user inputs for non-TH35 rails (TH35-15, asymmetric rails, etc.).
• Mechanism: rail slides in end-on; no spring/snap. Channel cross-section is built as hull(two thin Z-slabs) — the same idiom already used by wallmount.ts and utils.ts: roundedCube.
• Retention via undercut: mouth width = crown + clearance (27.5), chamber width = brim + clearance (35.5). The wider brim cannot pass back through the narrower mouth — a snap-in fit.
• Placement: bottom (default) and back. Each mode has its own translate/rotate so the channel exits to the correct face.
• Independent of wallMounts: flanges live on the left/right walls; the DIN boss lives on the bottom (or back) face — no geometric overlap.
• Defaults preserved: wallMounts stays at its current default; dinRailMount is opt-in (off by default) per the issue acceptance criteria.

Verification

• npm test → 28/28 SUCCESS (7 new + 21 existing).
• npm start → toggling Wall and DIN Rail independently and together in the Mounting accordion updates the live preview; switching Surface (with Height ≥ ~39 for Back) re-renders correctly; tweaking Rail Width / Crown Width / Depth / Length updates the slot.
• npm run format (prettier) — clean.
• npm run build — clean.

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