feat: add localUrl fallback with background reachability probe (#1313)

[Troublesis]

What this does

Adds an optional localUrl to dashboard items: an alternative URL (typically a LAN address) that is used only when it is reachable from the visitor's browser, otherwise the item falls back to the regular url.

This solves the long-standing local-vs-remote access problem described in #1313: when you reach Dashy over a VPN/Tailscale/remote tunnel, LAN addresses and local DNS don't resolve, so a single hard-coded URL can't serve both contexts. With localUrl, the same item "just works" on LAN (direct, fast) and remotely (falls back to the public/proxied URL) — no manual switching.

How it works

• On mount, if an item has a localUrl, Dashy probes it in the background with a no-cors fetch (we only care whether the request settles vs. aborts/errors — not its contents, so cross-origin LAN services don't trip CORS).
• The result drives a new effectiveUrl computed, which is already bound to the anchor href before the user clicks — so the probe never delays a click. Until the probe confirms reachability, the regular url is used.
• Re-probes when the tab regains visibility (e.g. the user changed networks), and optionally on a configurable interval.

New item options

Key	Type	Default	Description
localUrl	string	–	Alternative URL preferred when reachable from the browser
localUrlTimeout	number	1500	Probe timeout in ms, clamped 300–5000
localUrlCheckInterval	number	0	Seconds between background re-checks; 0 = only on load + tab focus, clamped to 300 max

All three are optional and fully backwards-compatible — items without localUrl behave exactly as before.

Changes

• src/mixins/ItemMixin.js — probe logic, effectiveUrl computed, timers/listener lifecycle
• src/components/LinkItems/Item.vue — bind effectiveUrl, start/stop probing on mount/unmount
• src/components/LinkItems/SubItem.vue — same lifecycle hooks for sub-items
• src/utils/config/ConfigSchema.json — schema for the three new properties
• docs/configuring.md — documents the new options

Example

- title: Home Assistant
  url: https://ha.example.com        # used when remote
  localUrl: http://192.168.0.10:8123 # preferred when on the LAN
  localUrlTimeout: 1500
  localUrlCheckInterval: 0

Notes

Closes the use case in #1313 (the issue was closed as not planned; opening this in case the maintainers reconsider, since the implementation is self-contained and opt-in). Timers and the visibilitychange listener are cleaned up in beforeUnmount to avoid leaks in multi-page navigation.

[lissy93]

Thanks @Troublesis
This looks great to me!

There were a few very small edits I just need made before it can be merged. Thought it would be quicker for me to do, but you've unticked the "Allow edits from maintainers" button in the sidebar.

So I'm going to fork of your branch and create a new PR over in #2207

(You'll still be credited as contributor)

[liss-bot]

Hey @Troublesis, this has now been implemented in #2207, and will be released shortly in 4.3.6 😇