feat(desktop): schema-driven memory-provider config surface

Make the desktop memory settings dynamic instead of hardcoded per
provider. The dropdown is now populated from discover_memory_providers()
(bundled + user-installed + pip) rather than a static enum, and each
provider's config panel is derived from its own get_config_schema() —
the same declaration `hermes memory setup` uses — so adding or porting a
provider is pure declaration with no bespoke UI, conditional, or
endpoint.

- memory_providers.py: reworked from a hand-written Hindsight registry
  into a pure adapter (describe_provider + coerce_value) that normalizes
  a provider's raw schema into typed fields — secret(+env), select,
  boolean, typed text — carrying `when` conditionals, url, and required.
- MemoryProvider ABC: add optional read_current_config() (default {}),
  the read-back mirror of save_config(). Ported mem0, hindsight, honcho,
  holographic; the rest fall back to schema defaults safely.
- web_server GET/PUT /api/memory/providers/{name}/config now load the
  live provider, derive its schema, write non-secrets via the provider's
  own save_config() (each keeps its native storage), persist secrets to
  the env store, and `when`-gate validation so hidden fields aren't
  required or written. Secrets stay write-only (is_set only).
- ProviderConfigPanel: `when`-conditional visibility (handles Hindsight's
  mode-gated duplicate keys), boolean toggle, and credential url links.
  Dropdown driven by getMemoryStatus(); hardcoded enum removed.

Tests assert the mapping contract and endpoint behavior (schema
derivation, save-via-save_config, secret-never-returned, when-gating,
select rejection) against real bundled providers rather than a snapshot
of a hardcoded list.

plugins/memory/mem0/__init__.py

@@ -166,6 +166,12 @@ class Mem0MemoryProvider(MemoryProvider):
             {"key": "rerank", "description": "Enable reranking for recall", "default": "true", "choices": ["true", "false"]},
         ]
 
+    def read_current_config(self):
+        try:
+            return _load_config()
+        except Exception:
+            return {}
+
     def _get_client(self):
         """Thread-safe client accessor with lazy initialization."""
         with self._client_lock: