docs(i18n): add Traditional Chinese (zh-TW) locale and READMEs

[maxmilian]

📝 Description

Add Traditional Chinese (Taiwan Mandarin) support across documentation and the frontend i18n system.

New files:

• docs/project/README.zh-tw.md — full Traditional Chinese README
• docs/project/CONTRIBUTING.zh-tw.md — contributor guide
• web/frontend/src/i18n/locales/zh-TW.json — UI strings (792 keys, parity with zh.json verified via jq)

Modified:

• web/frontend/src/i18n/index.ts — register zh-TW resource and add dayjs/locale/zh-tw import. Lower-case the incoming lng for the languageChanged handler so zh-TW / zh-Hant* / lower-cased variants land on Traditional dayjs formatting before the generic zh branch falls through to zh-cn.
• web/frontend/src/components/app-header.tsx, web/frontend/src/routes/launcher-{login,setup}.tsx — add 繁體中文 entry to the three language switcher dropdowns
• README.md + the 10 docs/project/README.<lang>.md files (including the new zh-tw.md) — add [繁體中文] to the language switcher row in canonical position (after Chinese), and rename the existing [中文] label to [简体中文] so the two Chinese variants are disambiguated (matches the dropdown labels which already use 简体中文 / 繁體中文)

Translation quality note: targets Taiwan Mandarin conventions (軟體, 檔案, 使用者, 螢幕, 伺服器, 儲存庫, 設定, 載入...) rather than mechanical simplified-to-traditional character conversion of zh.json / README.zh.md.

🗣️ Type of Change

• 🐞 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 📖 Documentation update
• ⚡ Code refactoring (no functional changes, no api changes)

🤖 AI Code Generation

• 🤖 Fully AI-generated (100% AI, 0% Human)
• 🛠️ Mostly AI-generated (AI draft, Human verified/modified)
• 👨‍💻 Mostly Human-written (Human lead, AI assisted or none)

AI drafted the three translation files; I curated the Taiwan-specific terminology, fixed the language-switcher line in the new README, wired up index.ts + the three dropdown components, ran an independent fresh-context code review (which surfaced the case-insensitivity hardening now applied), renamed [中文] → [简体中文] across all READMEs for disambiguation, and verified i18n key parity with jq.

🔗 Related Issue

None — greenfield translation. No prior issue or PR for zh-TW exists in the repo (verified via gh search code on zh-TW / zh-Hant / 繁體, all zero hits before this PR).

📚 Technical Context (Skip for Docs)

N/A — documentation/translation PR.

🧪 Test Environment

• Hardware: Apple MacBook Pro (M-series)
• OS: macOS 25.5.0 (Darwin)
• Model/Provider: N/A — UI/docs change, no agent runtime exercised
• Channels: N/A — no channel code touched

Verification performed

make lint-docs   # PASS — "docs lint: OK"

# i18n key parity (must be empty diff)
jq -r 'paths(scalars) | join(".")' web/frontend/src/i18n/locales/zh.json | sort > /tmp/k_zh.txt
jq -r 'paths(scalars) | join(".")' web/frontend/src/i18n/locales/zh-TW.json | sort > /tmp/k_zhtw.txt
diff /tmp/k_zh.txt /tmp/k_zhtw.txt
# (empty — 792 scalar keys in perfect parity)

# JSON validity
jq . web/frontend/src/i18n/locales/zh-TW.json > /dev/null   # valid

make check was also run. Two pre-existing test failures in pkg/tools (TestShellTool_DevNullAllowed, TestShellTool_FileURISandboxing) reproduce on a pristine upstream/main checkout — both fail with "path outside working dir" against macOS /var/folders/... symlinks, unrelated to this PR.

📸 Evidence (Optional)

Language switcher row after change (root README.md, line 21)

[简体中文](docs/project/README.zh.md) | [繁體中文](docs/project/README.zh-tw.md) | [日本語](docs/project/README.ja.md) | [한국어](docs/project/README.ko.md) | [Português](docs/project/README.pt-br.md) | [Tiếng Việt](docs/project/README.vi.md) | [Français](docs/project/README.fr.md) | [Italiano](docs/project/README.it.md) | [Bahasa Indonesia](docs/project/README.id.md) | [Malay](docs/project/README.ms.md) | **English**

☑️ Checklist

• My code/docs follow the style of this project.
• I have performed a self-review of my own changes.
• I have updated the documentation accordingly.

[CLAassistant]

All committers have signed the CLA.

[maxmilian]

Rebased onto latest upstream/main and resolved conflicts — the PR is now mergeable again (head 299ce023).

Conflicts resolved (2 files): upstream recently added the cs (Čeština) locale, which touched the same lines as this PR's zh-TW locale. Resolution was purely additive — both locales now coexist:

• web/frontend/src/i18n/index.ts — kept both the cs/zhTW imports and both resources entries
• web/frontend/src/components/app-header.tsx — kept both language-switcher dropdown items (繁體中文 + Čeština)

Verified with tsc --noEmit on web/frontend (0 errors). The other 11 README files auto-merged cleanly. Ready for review whenever you have a chance — thanks! 🙏