chore: add repo templates and normalize CI workflows (#1463) [no ci]

[harshithmohan]

Summary

Adds repository templates for PRs and issues, and normalizes the CI workflow files (kebab-case filenames, split lint stages). Supersedes #1462 (its branch was deleted).

• PR template consolidates the de-facto format from recent PRs (Summary → Changes with bolded file paths → optional Screenshots).
• Issue templates restore and modernize the version/channel fields the repo previously used (see Update to 5.3.1 (from 5.3.0) broke auto-matching #1290, WebUI Update Error 500 #1400): required server + WebUI versions and channels, since mismatched versions are the most common triage back-and-forth. Feature requests mirror the old template from Add Option to configure "Up Next" behaviour #1219. config.yml disables blank issues and routes support to Discord and server-side problems to ShokoServer.
• Workflow renames to lowercase kebab-case (matching codeql-analysis.yml/opencode.yml): Lint-PR.yml → validate-pr.yml (also renamed since it no longer just lints), Release-Dev-Auto.yml → release-dev-auto.yml, Release-Manual.yml → release-manual.yml, Update-Manifest.yml → update-manifest.yml. The two uses: references to update-manifest.yml were updated.
• Split lint stages in validate-pr.yml and release-manual.yml: the single pnpm lint --quiet step is now Format Check (pnpm dprint) → Lint (pnpm oxlint) → Stylelint (pnpm stylelint), so each failure reports separately.

Changes

• .github/PULL_REQUEST_TEMPLATE.md — new; Summary / Changes / Screenshots sections.
• .github/ISSUE_TEMPLATE/bug_report.yml — new; required verify-checklist, required version/channel fields, description, steps to reproduce, optional screenshots.
• .github/ISSUE_TEMPLATE/feature_request.yml — new; verify-checklist, description, optional examples/mockups.
• .github/ISSUE_TEMPLATE/config.yml — new; blanks disabled, Discord + ShokoServer contact links.
• .github/workflows/validate-pr.yml — renamed from Lint-PR.yml; lint stage split into Format Check / Lint / Stylelint before Test.
• .github/workflows/release-manual.yml — renamed from Release-Manual.yml; lint stage split the same way.
• .github/workflows/release-dev-auto.yml — renamed from Release-Dev-Auto.yml; update-manifest.yml reference updated.
• .github/workflows/update-manifest.yml — renamed from Update-Manifest.yml.
• AGENTS.md — workflow filename references updated.

Docs/CI-only change, [no ci].