feat(startup): validate config on every start; polish ini_writer appends

- meshcore_bot.py: run config validation on normal startup and print
  errors/warnings (capped at 15) — misspelled sections/keys previously
  failed silently, the top source of configuration confusion
- ini_writer: append new keys directly under a section's last entry
  instead of after its trailing blank line
- add .claude/launch.json for browser-preview of the standalone viewer

Verified end-to-end in the browser: /plugins renders 43 command cards,
opt-in commands show off, dynamic-row add/save writes config.ini with a
timestamped backup and preserved comments.

.claude/launch.json

@@ -0,0 +1,18 @@
+{
+  "version": "0.0.1",
+  "configurations": [
+    {
+      "name": "web-viewer",
+      "runtimeExecutable": ".venv/bin/python",
+      "runtimeArgs": [
+        "-m",
+        "modules.web_viewer.app",
+        "--config",
+        "/private/tmp/claude-501/-Users-adam-meshcore-bot/31648a90-50b9-4cae-b1f6-0e7815c5a855/scratchpad/viewer/config.ini",
+        "--port",
+        "8757"
+      ],
+      "port": 8757
+    }
+  ]
+}

meshcore_bot.py

@@ -113,6 +113,31 @@ def main():
                 print(f"Info: {message}", file=sys.stderr)
         sys.exit(1 if has_error else 0)
 
+    # Always sanity-check the config on normal startup too — misspelled
+    # sections/keys otherwise fail silently and are the most common source of
+    # "why doesn't my setting work" confusion. Non-fatal: warn and continue.
+    try:
+        from modules.config_validation import (
+            SEVERITY_ERROR as _SEV_ERR,
+            SEVERITY_WARNING as _SEV_WARN,
+            validate_config as _startup_validate,
+        )
+        _issues = [
+            (sev, msg) for sev, msg in _startup_validate(args.config)
+            if sev in (_SEV_ERR, _SEV_WARN)
+        ]
+        _MAX_SHOWN = 15
+        for sev, msg in _issues[:_MAX_SHOWN]:
+            print(f"Config {sev}: {msg}", file=sys.stderr)
+        if len(_issues) > _MAX_SHOWN:
+            print(
+                f"Config: ... and {len(_issues) - _MAX_SHOWN} more issue(s); "
+                "run with --validate-config for the full report",
+                file=sys.stderr,
+            )
+    except Exception as exc:  # noqa: BLE001 - never block startup on the linter itself
+        print(f"Config validation skipped: {exc}", file=sys.stderr)
+
     from modules.core import MeshCoreBot
     bot = MeshCoreBot(config_file=args.config)
 

modules/ini_writer.py

@@ -148,7 +148,12 @@ def update_ini_values(
 
     def _record_section_boundary() -> None:
         if current_norm is not None and current_norm in remaining:
-            section_body_end[current_norm] = len(out)
+            # Walk back over trailing blank lines so appended keys sit directly
+            # under the section's last entry instead of after the gap.
+            end = len(out)
+            while end > 0 and out[end - 1].strip() == "":
+                end -= 1
+            section_body_end[current_norm] = end
 
     for raw in lines:
         section_match = _SECTION_RE.match(raw)