Render server validation errors as form errors

[neo22neo]

Stacked on #420. This branch is based on fix/mbin-community-name-validation,
so until #420 merges this PR shows that commit and its files too. The only work
owned by this PR is the last commit, Render server validation errors as form errors (lib/src/api/client.dart + a small delta in
lib/src/screens/explore/community_owner_panel.dart). Once #420 is merged I'll
rebase so the diff here collapses to just that. Please merge #420 first.

Problem

#420 stops predictable bad names — anything outside Mbin's
^[a-zA-Z0-9_]{2,25}$ — before the request is ever sent. But the server still
rejects names for reasons a client cannot know in advance: already taken,
reserved, or blocked by instance policy. Those come back as 400 with an
RFC 7807-style body:

{
  "type": "https://tools.ietf.org/html/rfc2616#section-10",
  "title": "An error occurred",
  "status": 400,
  "detail": "..."
}

ServerClient.checkResponseSuccess embedded that entire JSON body into an
http.ClientException message, and the create screen had no try/catch, so
it reached the global handler in main.dart and rendered as a snackbar full of
raw JSON. The useful detail was buried inside it.

So: #420 handles malformed names before submit; this PR makes the server's own
validation responses render as form errors instead of raw
ClientException/JSON. It's the backstop, not the primary fix.

Changes

lib/src/api/client.dart

• New ServerErrorException carrying statusCode / uri / title / detail /
  rawBody, with toString() returning detail ?? title ?? <generic>.
• checkResponseSuccess now tries to parse an error body as a JSON object with
  a string title/detail and throws ServerErrorException when it can.
  Bodies that aren't structured problem JSON — Lemmy/PieFed {"error": ...},
  HTML, empty — still throw http.ClientException exactly as before. No call
  site in the repo matches ClientException by type, so this is backwards
  compatible and incidentally means every generic catch (e) now shows a clean
  message instead of a JSON blob.

lib/src/screens/explore/community_owner_panel.dart

• create() is wrapped in try / on ServerErrorException: a 400 carrying a
  detail is shown on the Name field through the errorText plumbing added in
  Validate Mbin community names when creating a community #420, and swallowed so no snackbar fires. Anything else is rethrown to the
  existing generic handling.
• The message is stored alongside the name it was returned for and only shown
  while the field still holds that name, so it clears itself on any edit —
  including Validate Mbin community names when creating a community #420's suggestion button, which sets the controller text directly
  and so never fires onChanged.
• The create/edit ternary became an if/else so only the create call is
  wrapped; the edit path is behaviourally unchanged.

This PR deliberately contains no validation, helper-text, suggestion, or
TextEditor changes — all of that is #420's.

Testing

No Flutter toolchain in this environment, so dart format / flutter test /
the app itself were not run locally — CI is the first real check. Verified by
hand: the sample body above parses with detail as a String, and no line in
the changed files exceeds 80 columns.

Manual test plan (Mbin account), after #420:

1. Name that Validate Mbin community names when creating a community #420 already rejects → inline error, submit blocked, no request.
2. Well-formed name that the server rejects anyway (e.g. one already taken) →
   the server's detail appears under the Name field, no snackbar.
3. Edit the name → the server error clears; tap Validate Mbin community names when creating a community #420's suggestion button → it
   also clears.
4. Force an unrelated failure (offline, 500) → still the generic snackbar.
5. Lemmy/PieFed creation and the Mbin community edit screen unchanged.

🤖 Generated with Claude Code

https://claude.ai/code/session_015icyWnYYmR7J8oSHocta6U

[neo22neo]

@jwr1 could you take a look when you have a chance? This turns the Mbin 400 name-validation response into an inline field error instead of the raw-JSON snackbar. I couldn't run dart format locally (no Flutter toolchain here), so if the analysis job flags whitespace I'll push a formatting-only fixup.

[neo22neo]

Restructured this PR: it was independently duplicating #420's client-side validation and TextEditor changes. It's now stacked on fix/mbin-community-name-validation and reduced to only the server-error handling — ServerErrorException in api/client.dart plus a small delta in the create screen that routes a 400's detail through the errorText plumbing #420 adds. All duplicated validation/helper/TextEditor code has been removed. Please merge #420 first; I'll rebase afterwards so the diff here collapses to just the last commit.