feat: allow empty submit when selected context is attached

[quiet-node]

Description

Pressing Enter on an empty ask bar now sends a turn whenever Thuki has captured selected text from the app you came from. The selection alone is the message, and the system prompt decides what happens to it, so a persona that already says "analyse and explain the content" no longer needs a typed instruction every time. Closes #363.

Key changes

• handleSubmit resolves the sanitized selection up front and treats it as content, so an empty ask bar with a live selection is a valid submit. A genuinely empty submit, with no text, no images and no selection, stays blocked.
• The "nothing to send" gate generalizes from a /think-only exception to any selection, stating the rule once instead of twice.
• The ask bar's submit button enables on a live selection, so the button and the Enter key agree.
• runChatTurn no longer bails on a blank message when quoted text is present.
• build_user_content composes the user message content. With no typed request it emits the [Highlighted Text] block alone and omits the [Request] header, leaving the system prompt as the sole authority over the content. It sits outside ask_model so the branch is covered by tests rather than by the coverage-off command body.
• The system prompt's Input Contract gains the matching fallback: highlighted text with no request means the selection is the subject, and the model engages it directly instead of asking what to do with it.
• A selection-only submit clears sticky replace mode. Without that, a deferred /rewrite whose image failed could leave sticky armed alongside a live selection, and the next bare submit would auto-replace the user's selection in the source app with a free-form answer.
• Built-in web search stays available on a selection-only turn, but the classifier decides it rather than the pre-filter's deterministic shortcut. That shortcut forces a search on a URL, a current-or-future year, or a word like "latest", which reads intent correctly only when the user wrote the words. On highlighted prose those tokens belong to the page's author, so has_user_request withholds the shortcut and hands the turn to the model. Turns carrying a typed request are unaffected, and /search still forces a search from its own path.

Testing

bun run test:all:coverage and bun run validate-build both green: 2652 frontend tests, 2319 backend tests, 100% coverage on both sides.