chore: refresh dependencies and align with official plugin conventions

[roberson-io]

Closes #85.

Summary

Refresh Go and webapp dependencies (dormant since v1.3.2), bring tooling and config into line with the actively-maintained Mattermost plugins (GitHub / GitLab / Jira), and clear the security debt that was blocking a release cut.

• Go: 1.21 → 1.25.8; mattermost/server/public v0.0.12 → v0.3.0; all indirects refreshed
• Webapp: React 16 → 18, Webpack 4 → 5, Jest 27 → 29, Babel 7.16 → 7.29, TypeScript 4.6 → 5.7, ESLint 6 → 8.57; mattermost-redux 5.33 → 10.9; added @mattermost/client / @mattermost/types / react-dom / @testing-library/*; removed dead deps (superagent, enzyme+kin, react-intl, babel-eslint, @emotion/babel-preset-css-prop)
• Tooling / config: .nvmrc 16 → 24; golangci-lint v1.51.1 → v2.9.0 (config migrated to v2 format); plugin.json min_server_version 5.11 → 10.7, arm64 executables declared
• Latent fixes surfaced by newer toolchain: non-const format strings, unchecked defer *.Close(), gosec G304 on trusted paths, misspell, staticcheck QF1012; also fixes a pre-existing flaky TestFilterLogEntries in build/pluginctl/

Vulnerability delta

Scan	Before	After
govulncheck (reachable)	2	0
npm audit	98 (12 critical, 39 high)	0

Test plan

• make check-style passes (golangci-lint v2.9.0 + webapp lint + type check)
• make test passes (Go + webapp)
• make dist produces a valid bundle
• go mod tidy and make apply are both idempotent (CI diff checks pass)
• Load the built plugin against a current Mattermost server (≥ 10.7) and exercise /webex commands

[avasconcelos114]

LGTM, Awesome job with the upgrade!