chore(repo): fix license detection and add github workflows

LICENSE now starts with the EUPL-1.2 standard header so github-linguist
detects the licence correctly in the repo header. The dual-copyright
block (upstream ChatTwo authors plus Hellion Online Media) moves into a
new COPYRIGHT file referenced from the README. NOTICE.md and
UPSTREAM_SYNC.md stay as-is.

New files under .github:

- workflows/build.yml: validates every push to main and every PR
  against the current Dalamud staging branch on a Windows runner
- workflows/release.yml: builds Release on every v* tag, locates the
  DalamudPackager latest.zip and attaches it to the matching GitHub
  Release via softprops/action-gh-release
- dependabot.yml: weekly NuGet sweeps and monthly GitHub Actions
  sweeps with conventional-commit prefixes, grouped patch and minor
  PRs to cut review noise
- ISSUE_TEMPLATE/bug_report.yml + feature_request.yml + config.yml:
  structured intake that pushes security reports through the private
  advisory flow and routes upstream-only issues to ChatTwo
- SECURITY.md: documents the vulnerability reporting channels, scope,
  and target disclosure window

The release workflow replaces the previous manual upload step. Tag a
release and the ZIP shows up on the release page automatically.

LICENSE

@@ -1,31 +1,5 @@
-HellionChat — a privacy-focused fork of ChatTwo
-
-Copyright (c) 2024-2025 Infiziert90 (Infi) and Anna Clemens
-    (original ChatTwo authors and copyright holders of the upstream work
-    this fork is built on)
-Copyright (c) 2025-2026 Florian Wathling / Hellion Online Media
-    (HellionChat-specific modifications, Privacy/Export/Auto-Tell-Tabs
-    features, Hellion theme, German localisation, EUPL-1.2 fork
-    maintenance)
-
-Licensed under the European Union Public Licence (EUPL), Version 1.2 only.
-
-You may obtain a copy of the Licence at:
-https://eupl.eu/1.2/en/
-
-Unless required by applicable law or agreed to in writing, software
-distributed under the Licence is distributed on an "AS IS" basis, WITHOUT
-WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
-Licence for the specific language governing permissions and limitations
-under the Licence.
-
-The full Licence text follows below.
-
-================================================================================
-
-EUROPEAN UNION PUBLIC LICENCE v. 1.2
-
-EUPL (c) the European Union 2007, 2016
+                      EUROPEAN UNION PUBLIC LICENCE v. 1.2
+                      EUPL (c) the European Union 2007, 2016
 
 This European Union Public Licence (the 'EUPL') applies to the Work (as
 defined below) which is provided under the terms of this Licence. Any use
@@ -37,7 +11,7 @@ The Work is provided under the terms of this Licence when the Licensor (as
 defined below) has placed the following notice immediately following the
 copyright notice for the Work:
 
-    Licensed under the EUPL
+        Licensed under the EUPL
 
 or has expressed by any other means his willingness to license under the
 EUPL.