feat: [2267] Dashboard data store - Report store write failures

[n-lark]

Description

First layer of #2239. Before the store starts driving the dashboard, failures in it need to be visible: today every store write swallows its errors, so a store that stops accepting writes fails silently.

• Failed store writes and clears now warn on the widget that hit them, once per deploy, so the debug sidebar shows what broke without a warning per message.
• Warnings name the real cause instead of an internal error.
• A context store that saves copies no longer reports "replaced by a flow" on every write. That warning now only fires when the store really was replaced.

Test plan

• Break the global context mid-run: one warning on the widget, the dashboard keeps updating
• Redeploy, break it again: one new warning
• Remove a widget while the context is broken: one "clearing a store entry failed" warning
• A flow replacing global.dashboardStore still warns once
• A context store that saves copies gives one write warning, not a "replaced by a flow" warning per write

Related Issue(s)

Resolves #2267

Checklist

• I have read the contribution guidelines
• Suitable unit/system level tests have been added and they pass
• Documentation has been updated
  • Upgrade instructions
  • Configuration details
  • Concepts
• Changes flowforge.yml?
  • Issue/PR raised on FlowFuse/helm to update ConfigMap Template
  • Issue/PR raised on FlowFuse/CloudProject to update values for Staging/Production
• Link to Changelog Entry PR, or note why one is not needed.

Labels

• Includes a DB migration? -> add the area:migration label

[n-lark]

Sooo warnOnce only warns on the first failure. After that, every store failure, from any widget, is silent until the next full deploy or restart. Could extend it to warn per type of failure, or every N failures. Lmk if we want that, can always iterate down the line.

[cstns]

fine as is

[cstns]

Tested with a store that returns copies mid run, one clear warning instead of one per write and the dashboard keeps updating.