#2422 web: the PCIe metric shows the current link generation, not the card's capability

Description

## Summary

The Monitor tab's PCIe metric showed the maximum generation the card supports (`gpu_pcie_gen_max`), and when the live generation was lower it said `idle GenN` - which reads as a power-saving state. A downgraded link does not reach the max under load either (a x8 link, a power cap, a bad riser), so the metric now shows the **current** generation and says `below its GenN capability` when the two differ.

## What it changes and what it leaves alone

Changes only the one metric line in `serve/web/app.js`. The sparklines, every other card and the server are untouched.

## How it was checked

Display-only. Observed against `nvidia-smi`'s current link on the box in #2099 (Gen1 / Gen3 x8 / Gen3 x16 tiers): the card reports a Gen3 capability while the link runs below it under the power cap, and the old wording presented the capability as the link.

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