fix(i18n): refine inaccurate and non-Taiwan zh-TW UI copy

## Problem

`frontend/resources/translations/zh-TW.json` contains a set of inaccurate, malformed, or regionally inconsistent UI strings. Representative examples:

- `sideBar.upgradeToAILocalDesc` repeats the title instead of translating “Chat with PDFs, improve your writing, and auto-fill tables using local AI”.
- `approveAccess.alreadyProMessage` renders an extra `>` after the `` placeholder.
- `settings.cancelSurveyDialog.questionTwo.answerThree` contains the Simplified Chinese character in `不确定`.
- Several user-facing strings use non-Taiwan terms such as `用戶`, `網絡`, `圖標`, `鏈接`, `代碼`, and `優先級`.
- Authentication messages have awkward wording and one `己`/`已` typo.

These strings appear across sign-in, workspace, local AI, billing, editor, template, invitation, and access-request surfaces.

## Proposed scope

Review the affected values against the English source and their UI context, then update only the canonical `zh-TW.json` file. Preserve all keys, data types, interpolation placeholders, references, tags, and intentional newlines.

The review excludes keys already covered by open PRs #8815, #8907, and #8915. PR #8670 is an older broad translation sync with a conflicting and substantially incomplete locale snapshot; it does not contain these targeted corrections.

## Assistance disclosure

This localization review and issue draft were assisted by OpenAI Codex. Each proposed change is being checked against the English source, JSON structure, placeholder parity, and Taiwan usage before submission.