app/vlagent: implement /targets page

[vadimalekseev]

This improves the user experience when using vlanget to collect Kubernetes logs and files.

This commit uses the bootstrap CSS library for the VictoriaMetrics navbar, ensuring a consistent look and feel similar to vmagent. While the overall design currently differs from vmagent, we can align them further later if required.

This PR increases binary size of vlagent from 18648322 bytes to 19015666 (plus ~300 KiB).

Updates #1391

See the /targets web page screenshots bellow:

[cubic-dev-ai]

2 issues found across 16 files

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="docs/victorialogs/CHANGELOG.md">

<violation number="1" location="docs/victorialogs/CHANGELOG.md:29">
P1: Custom agent: **Technical Writer Review Agent**

Changelog entry contains multiple typos and a grammar error: 'shold' (should), 'vlanget' (vlagent), and incorrect verb form 'should improves' (should improve) on the same line</violation>
</file>

<file name="app/vlagent/filecollector/processor.go">

<violation number="1" location="app/vlagent/filecollector/processor.go:80">
P1: `/targets` can panic when `-fileCollector.fileField` is set to a custom name, because debug info no longer guarantees a `file` field.</violation>
</file>

_{Reply with feedback, questions, or to request a fix. Tag @cubic-dev-ai to re-run a review, or fix all with cubic.}