Fix: stop repeating the cloud subscription notice on model switches

[DevMando]

Summary

Picking a cloud model announced that cloud models need an ollama.com subscription, and kept
announcing it. The notice is removed — the model chip already says cloud, and the only message
that was ever actionable already appears when a cloud request actually fails.

What was happening

The notice was supposed to appear once. Two separate defects meant it appeared far more often than
that for some people, and never at all for others.

The guard was per-agent, not app-wide. The "already said this" flag was an instance field on
ChatController, and there is one controller per agent tab. The fact being stated is app-wide —
cloud needs a subscription — but the memory of having stated it lived in a single tab. Four agents
open meant four announcements.

The flag was burned without showing anything. It was set before the check that suppresses
announcements during restore, so a tab restored onto a cloud model marked the notice as shown while
displaying nothing — and then suppressed it for the rest of that session. So the behaviour was not
merely chatty, it was inconsistent in both directions.

What changed

• The notice is removed, not repaired. It fired on selection, pre-emptively, in case a 403
  arrived later. But the model chip already marks a cloud model, and ResponseStreamer already
  reports the useful version if a request actually fails — that the account is signed in but has no
  active subscription. The removed notice told the user something they could not act on, before
  anything had gone wrong.
• Replay still recognises it, and now covers the older wording too. Nothing emits it any more,
  which means journal replay is now the only thing keeping it off screen for anyone whose journal
  already holds one. The 0.14.x wording contains an apostrophe and is journaled HTML-encoded, so the
  filter now decodes before matching — comparing encoded text against a raw literal fails silently,
  which is precisely how it would have reappeared on restore.
• The setup wizard is untouched. It explains cloud versus local while you are choosing a starter
  model, and mentions the subscription only when a sign-in check has actually failed, immediately
  before offering to sign you in. Both are conditional and actionable, unlike the removed one.

Scope and risk

Low. One notice deleted, one field deleted, and the replay filter widened. No behaviour changes
beyond what is displayed.

The judgement worth reviewing is whether losing the pre-emptive warning matters. The argument for
removing it: a user who picks a cloud model without a subscription now learns at their first
message instead of at selection — one step later, but with a message that names the actual problem
and is tied to a real failure rather than a hypothetical one. If you would rather keep a warning at
selection time, the alternative was a once-ever flag persisted to ui-settings.json; happy to build
that instead.

Verification

• dotnet build src/MandoCode.Desktop — succeeded, 0 warnings, 0 errors.
• dotnet test src/MandoCode.Desktop.Tests — 490 passed, 0 failed.
• New coverage: both retired wordings are stripped from a replayed journal, including the
  HTML-encoded 0.14.x one — that test fails without the decode, so it pins the actual bug rather
  than restating the code.
• Also pinned: the wizard's own cloud explainer is a dim notice mentioning the same subscription and
  must survive replay, since it is part of a walkthrough the user went through. The filter
  matches the exact retired notices rather than anything containing the word "cloud".

Not covered: no runtime check that a real cloud switch is now silent, or that a real 403 still
produces the actionable message — the 403 path is unchanged by this PR but is the thing a user now
depends on. Worth switching to a cloud model in a build of this branch and confirming you get the
chip and nothing else.