#3158 web: the Monitor headlines the PCIe link's current generation (#2137)

Beschreibung

## What it changes

`serve/web/app.js`, the PCIe metric on the Monitor:

- the headline is now `gpu_pcie_gen` — the link's **current** generation — falling back to `gpu_pcie_gen_max` when the current value is absent;
- when the current generation is below the capability, the note says `· below its GenN capability` instead of `· idle GenN`.

The old code headlined the capability and called a below-capability link "idle", which on a link whose Gen3 training never ran reads "Gen3 · idle Gen1" — exactly the opposite of the diagnostic that matters. One sample cannot tell idle power saving from a permanent downgrade, so the new note stays neutral.

## What it leaves alone

Only the rendering of the PCIe card; the telemetry fields and every other card are unchanged.

## How I checked

jsdom, loading the real `serve/web/index.html` + `app.js` with `/metrics` stubbed:

| `gpu_pcie_gen` | `gpu_pcie_gen_max` | headline | note |
|---|---|---|---|
| 1 | 3 | `Gen1x16` | `to GPU 3191 MB/s · below its Gen3 capability` |
| 3 | 3 | `Gen3x16` | `to GPU 0,0 MB/s` |
| 1 | – | `Gen1x16` | `to GPU 10 MB/s` |
| – | 3 | `Gen3x16` | `to GPU 10 MB/s` |
| – | – | `–` | |

Reported by @MrKop-git in #2137, whose root-cause analysis and suggested rendering this follows.

🤖 Generated with [Claude Code](https://claude.com/claude-code)