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.