Show CPU in cores at 1 core and above (64 instead of 64000m)

[TomerKarniol]

Problem

CPU is always shown in millicores, while memory switches to Gi. In :nodes on a large node, memory reads 251.4Gi but CPU reads 64000m. That's hard to scan, and the two columns follow different rules.

fmt_cpu in src/columns.rs always formats as {milli}m. docs/views.md describes this on purpose ("4 becomes 4000m"), so I'm asking before changing it.

Proposal

Format CPU the way Kubernetes writes quantities, and the way kubectl top nodes shows them:

Value	Today	Proposed
250m	250m	250m
1000m	1000m	1
1500m	1500m	1.5
1234m	1234m	1.23
64000m	64000m	64

• Below 1 core: whole millicores, as now.
• From 1 core up: cores with at most two decimal places and no trailing zeros. There is no unit suffix, because bare numbers are valid Kubernetes CPU quantities and sofka's filters already read them as cores (cpu>2).

Scope

• The change goes in fmt_cpu, so it covers the node and pod CPU columns, the right-size view, and the container trend scale in src/ui.rs.
• Sorting and filters use raw millicores and don't change.
• Tests and the docs that describe the format (docs/views.md, docs/features.md) would be updated in the same PR.