fix(editor): unit-aware sidebar controls + preserve user-typed precision

[b9llach]

What does this PR do?

Two related fixes to the sidebar's length inputs.

1. SliderControl is unit-aware

useViewer.unit ('metric' | 'imperial') already existed and was toggled by the toolbar m/ft button, but the only places that actually respected it were wall-measurement-label, site-edge-labels, metric-control, and the 2D floorplan-panel. Every sidebar slider hard-coded unit="m" as its suffix and showed raw metres regardless of the user's preference. This PR teaches SliderControl to read useViewer.unit and, when the caller passes unit="m", convert the value for display + editing while keeping scene data in metres (conversion is display-only). About 70 slider instances across every panel pick up imperial support with zero call-site changes.

Sliders with non-"m" unit strings (e.g. "%", "°") are unchanged.

2. Parent-side Math.round(...) clipping removed from panels

Every panel had a value={Math.round(node.X * 100) / 100} pattern around its sliders. It was there to defend against floating-point display noise (2.10000000000000001 → "2.10"), but SliderControl's own .toFixed(precision) already does that — so the parent rounds were redundant even in metric mode.

With the imperial path landed, the parent rounds became actively harmful. Tracing the door-height precision={2} case:

1. User sees "8.01 ft" (whatever was stored), types 8.00, Enter
2. submitValue converts: 8 / 3.28084 ≈ 2.43840 m
3. onChange(2.43840) — handler calls updateNode({ height: 2.43840 })
4. React re-renders; door-panel.tsx passes value={Math.round(node.height * 100) / 100} = 2.44
5. SliderControl gets 2.44, displays toDisplay(2.44).toFixed(2) = (2.44 × 3.28084).toFixed(2) = "8.01"

So typing 8.00 bounced back to 8.01. Removing the Math.round in the parent lets the full-precision metres value survive the round trip, and the display cleanly shows "8.00" again.

The submitValue path also now rounds in the display unit before converting back to metres, so user-typed values preserve their exact precision regardless of which unit is stored underneath.

New shared helper

packages/editor/src/lib/units.ts — formatLength, formatArea, metersToFeet, feetToMeters, METERS_TO_FEET. The slab + ceiling Info sections now use formatArea(area, unit) instead of a hardcoded "m²" suffix. Follow-up opportunity (out of scope for this PR): consolidate the four existing ad-hoc copies in wall-measurement-label.tsx, site-edge-labels.tsx, metric-control.tsx, and floorplan-panel.tsx onto this new module.

How to test

1. bun dev
2. Click the m/ft toggle in the viewer toolbar.
3. Click a door: every slider suffix should flip from m to ft, and the numbers should reflect the conversion (2.1 m height shows as 6.89 ft).
4. Type 8.00 into any length slider while in imperial mode, press Enter: value should stay at 8.00 ft (on main it snaps to 8.01 ft).
5. Flip back to metric: everything returns to metres, no precision loss.
6. Click a slab, check the Info section: area should read "12.50 m²" in metric and "134.55 ft²" in imperial.

Screenshots / screen recording

N/A — unit-toggle behaviour, hard to screenshot without video. Will record a short clip if reviewers want one.

Checklist

• I've tested this locally with bun dev
• My code follows the existing code style (bun check passes on the touched files — verified via biome check at @biomejs/biome@^2.4.6, which is the version pinned in root package.json)
• I've updated relevant documentation (N/A — no docs affected)
• This PR targets the main branch